NPE when element schema is not defined in persistence.json instead of using default schema names

[holger-stenzhorn]

Describe the bug
If no element schema defined is defined in persistence.json then one receives a NPE when running Docker `fhir-persistence-schema.

To Reproduce

1. Create a file persistence.json with the following content, i.e. no element schema defined:

   {
       "persistence": [
           {
               "db":  {
                   "host": "db",
                   "port": "5432",
                   "database": "fhirdb",
                   "user": "postgres",
                   "password": "change-password",
                   "type": "postgresql",
                   "ssl": "false",
                   "certificate_base64": "empty"
               },
               "behavior": "onboard"
           }
       ]
   }

2. Run docker run --env ENV_TOOL_INPUT=$(cat persistence.json | base64) ibmcom/ibm-fhir-schematool:latest
3. See error:

   ibm-fhir-schematool_1  | + /opt/java/openjdk/bin/java -jar /opt/schematool/fhir-persistence-schema-4.5.2-cli.jar --prop db.host=postgres --prop db.port=5432 --prop db.database=fhir --prop user=fhir --prop password=change-password --db-type postgresql --create-schema-oauth null --create-schema-batch null --create-schema-fhir null --pool-size 2
   ibm-fhir-schematool_1  | + tee out.log
   ibm-fhir-schematool_1  | 2020-12-08 12:37:53.830 00000001    INFO   com.ibm.fhir.schema.app.Main Opening connection to: jdbc:postgresql://postgres:5432/fhir
   ibm-fhir-schematool_1  | 2020-12-08 12:37:54.115 00000001    INFO s.postgresql.PostgreSqlAdapter The schema 'FHIR_ADMIN' is created or already exists
   postgres_1             | 2020-12-08 12:37:54.117 UTC [75] ERROR:  syntax error at or near "NULL" at character 29
   postgres_1             | 2020-12-08 12:37:54.117 UTC [75] STATEMENT:  CREATE SCHEMA IF NOT EXISTS NULL
   ibm-fhir-schematool_1  | 2020-12-08 12:37:54.129 00000001  SEVERE   com.ibm.fhir.schema.app.Main schema tool failed
   ibm-fhir-schematool_1  | 
   ibm-fhir-schematool_1  | org.postgresql.util.PSQLException: ERROR: syntax error at or near "NULL"
   ibm-fhir-schematool_1  |   Position: 29
   ibm-fhir-schematool_1  | com.ibm.fhir.database.utils.api.DataAccessException: org.postgresql.util.PSQLException: ERROR: syntax error at or near "NULL"
   ibm-fhir-schematool_1  |   Position: 29
   ibm-fhir-schematool_1  | 	at com.ibm.fhir.database.utils.postgresql.PostgreSqlTranslator.translate(PostgreSqlTranslator.java:104)
   ibm-fhir-schematool_1  | 	at com.ibm.fhir.database.utils.common.JdbcTarget.runStatement(JdbcTarget.java:48)
   ibm-fhir-schematool_1  | 	at com.ibm.fhir.database.utils.common.CommonDatabaseAdapter.runStatement(CommonDatabaseAdapter.java:230)
   ibm-fhir-schematool_1  | 	at com.ibm.fhir.database.utils.postgresql.PostgreSqlAdapter.createSchema(PostgreSqlAdapter.java:304)
   ibm-fhir-schematool_1  | 	at com.ibm.fhir.schema.app.Main.createSchemas(Main.java:300)
   ibm-fhir-schematool_1  | 	at com.ibm.fhir.schema.app.Main.process(Main.java:1438)
   ibm-fhir-schematool_1  | 	at com.ibm.fhir.schema.app.Main.main(Main.java:1513)
   ibm-fhir-schematool_1  | Caused by: org.postgresql.util.PSQLException: ERROR: syntax error at or near "NULL"
   ibm-fhir-schematool_1  |   Position: 29
   ibm-fhir-schematool_1  | 	at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2533)
   ibm-fhir-schematool_1  | 	at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2268)
   ibm-fhir-schematool_1  | 	at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:313)
   ibm-fhir-schematool_1  | 	at org.postgresql.jdbc.PgStatement.executeInternal(PgStatement.java:448)
   ibm-fhir-schematool_1  | 	at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:369)
   ibm-fhir-schematool_1  | 	at org.postgresql.jdbc.PgStatement.executeWithFlags(PgStatement.java:310)
   ibm-fhir-schematool_1  | 	at org.postgresql.jdbc.PgStatement.executeCachedSql(PgStatement.java:296)
   ibm-fhir-schematool_1  | 	at org.postgresql.jdbc.PgStatement.executeWithFlags(PgStatement.java:273)
   ibm-fhir-schematool_1  | 	at org.postgresql.jdbc.PgStatement.executeUpdate(PgStatement.java:247)
   ibm-fhir-schematool_1  | 	at com.ibm.fhir.database.utils.common.JdbcTarget.runStatement(JdbcTarget.java:45)
   ibm-fhir-schematool_1  | 	... 5 more
   ibm-fhir-schematool_1  | 2020-12-08 12:37:54.137 00000001  SEVERE   com.ibm.fhir.schema.app.Main SCHEMA CHANGE: RUNTIME ERROR
   

Expected behavior
The default schema names, i.e. FHIRDATA, FHIR_OAUTHand FHIR_JBATCH should be used.

Additional context
n/a