Media: Fix logic when checking IPTC alt text meta data.

Fix two conditional checks for validity of data when importing alt text from IPTC meta data. Strictly compare the results of `strpos()` as a boolean, rather than treating 0 as false; return string if `file_get_contents()` returns false.

Props suhan2411, pbiron, alexodiy, joedolson.
Fixes #64849.

git-svn-id: https://develop.svn.wordpress.org/trunk@62027 602fd350-edb4-49c9-b593-d223f7449a82