test: Verify string() function invocation

Add new test cases for string and null arguments.

(cherry picked from commit b56d41b)

src/test/scala/org/camunda/feel/impl/builtin/BuiltinConversionFunctionsTest.scala

@@ -184,6 +184,10 @@ class BuiltinConversionFunctionsTest extends AnyFlatSpec with Matchers with Feel
     eval(""" string(1.1) """) should be(ValString("1.1"))
   }
 
+  it should "convert a string" in {
+    eval(""" string("hello") """) should be (ValString("hello"))
+  }
+
   it should "convert Boolean" in {
 
     eval(""" string(true) """) should be(ValString("true"))
@@ -250,6 +254,10 @@ class BuiltinConversionFunctionsTest extends AnyFlatSpec with Matchers with Feel
     eval(""" string(@"P1Y2M") """) should be(ValString("P1Y2M"))
   }
 
+  it should "return null if the argument is null" in {
+    eval(""" string(null) """) should be (ValNull)
+  }
+
   "A duration() function" should "convert day-time-String" in {
 
     eval(""" duration(x) """, Map("x" -> "P2DT20H14M")) should be(ValDayTimeDuration("P2DT20H14M"))