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

scripts/symbols/generate_ctags_symbols.php

@@ -39,6 +39,10 @@ $data = array();
 $futures = array();
 
 foreach (explode("\n", trim($input)) as $file) {
+  if (!strlen($file)) {
+    continue;
+  }
+
   $file = Filesystem::readablePath($file);
   $futures[$file] = ctags_get_parser_future($file);
 }

scripts/symbols/generate_php_symbols.php

@@ -27,6 +27,10 @@ $data = array();
 $futures = array();
 
 foreach (explode("\n", trim($input)) as $file) {
+  if (!strlen($file)) {
+    continue;
+  }
+
   $file = Filesystem::readablePath($file);
   $data[$file] = Filesystem::readFile($file);
   $futures[$file] = PhutilXHPASTBinary::getParserFuture($data[$file]);