R1:ba25586016b2

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.