test: Verify number() return null

Add a test case to verify that the function number() returns null if the given string is not a number.

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

@@ -16,6 +16,7 @@
  */
 package org.camunda.feel.impl.builtin
 
+import org.camunda.feel.api.EvaluationFailureType.FUNCTION_INVOCATION_FAILURE
 import org.camunda.feel.api.FeelEngineBuilder
 import org.camunda.feel.impl.interpreter.MyCustomContext
 import org.camunda.feel.impl.{EvaluationResultMatchers, FeelEngineTest}
@@ -218,6 +219,30 @@ class BuiltinConversionFunctionsTest
     )
   }
 
+  it should "return null if the string is not a number" in {
+
+    evaluateExpression(""" number("x") """) should (
+      returnNull() and reportFailure(
+        FUNCTION_INVOCATION_FAILURE,
+        "Failed to invoke function 'number': Can't parse 'x' as a number"
+      )
+    )
+
+    evaluateExpression(""" number("x", ".") """) should (
+      returnNull() and reportFailure(
+        FUNCTION_INVOCATION_FAILURE,
+        "Failed to invoke function 'number': Can't parse 'x' as a number"
+      )
+    )
+
+    evaluateExpression(""" number("x", ".", ",") """) should (
+      returnNull() and reportFailure(
+        FUNCTION_INVOCATION_FAILURE,
+        "Failed to invoke function 'number': Can't parse 'x' as a number"
+      )
+    )
+  }
+
   "A string() function" should "convert Number" in {
 
     evaluateExpression(""" string(1.1) """) should returnResult("1.1")