Incorrect exception in the Merge MapJoinTask into its child MapRedTask optimizer

The CommonJoinTaskDispatcher#mergeMapJoinTaskIntoItsChildMapRedTask method throws a SemanticException if the number of {{FileSinkOperator}}s it finds is not exactly 1. The exception is valid if zero operators are found, but there can be valid use cases where multiple FileSinkOperators exist.

Example: the MapJoin and it child are used in a common table expression, which is used for multiple inserts.