Improve symbol generation scripts

Summary: Currently the symbol generation scripts fail if passed a list containing no files because explode("\n", $input) returns array("") rather than array(). This means that a generic Harbormaster Build Plan with a step which executes find . -type f -name '*.php' | ./scripts/generate_php_symbols.php won't work because it fails in repositories that don't contain any PHP code.

Test Plan: Ran echo | generate_php_symbols and saw no output instead of an exception.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19588