Support SQL that does not contain table names, such as SELECT 1

[dexter9000]

Please answer these questions before submitting your issue. Thanks!

Which version of Sharding-Jdbc do you using?

1.5.3

Expected behavior

在spring boot注入数据源时，org.springframework.boot.actuate.health.DataSourceHealthIndicator会定时执行"SELECT 1"语言监测数据源的连通性
对于不指定表名的数据库访问能够直接访问默认数据源

Actual behavior

抛出异常Cannot find table rule and default data source with logic tables: '[]'
导致spring的健康检查认为失败

db: {
status: "DOWN",
primaryDataSource: {
status: "UP",
database: "H2",
hello: 1
},
multipleDS: {
status: "DOWN",
database: "H2",
error: "com.dangdang.ddframe.rdb.sharding.exception.ShardingJdbcException: Cannot find table rule and default data source with logic tables: '[]'"
}
}

[terrymanu]

fixed at 2.0.0.m1