Don't pass empty strings to strpos

When an empty parameter is passed to a function using strpos, such as when
an extra comma appears at the end of contains_any's parameter list, don't
call strpos on the empty string.

Bug: 60203
Change-Id: I6221a01ad1ec9090de7bfc1d9d6583f22ba0eb2e
Merge "Don't pass empty strings to strpos"