Scenario: UNION result handling

Previous Behavior: queries in which union operator was used may have created an invalid output type.for example, the column type is ambigous in the following query:

select cast(1 as int) union select cast(1 as string)

Selecting the inappropriate type may cause the value to be changed to NULL.

New Behavior: The types are checked prior to execution; and the ambigous cases are rejected;

FAILED: SemanticException Schema of both sides of union should match: Column _c0 is of type int on first table and type string on second table. Cannot tell the position of null AST. (state=42000,code=40000)

The query should be clarified with explicit casts.