Fix FE bug from missing casts for functions with wildcard decimal args.

Before, if none of the child args where decimals, the casts would be added.
This is because getResolvedWildcardDecimalType could return a non-null, non-decimal
type.

Change-Id: I652c3afc00ce4c2047660dc4d226b299a11069a6
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2507
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins