Fix erroring on a text editor with no path

Under certain circumstances, it might be possible that Linter calls us on a text editor with no path. See #234 and https://github.com/AtomLinter/linter-eslint/blob/9eb5bbb2c771394b6bbe6aef172a4636cece9c4b/src/main.js#L220-L224.

lib/linter-js-standard.js

@@ -47,6 +47,10 @@ module.exports = function (textEditor) {
   var fileScope = textEditor.getGrammar().scopeName
   var config = atom.config.get('linter-js-standard')
 
+  if (!filePath) {
+    return null
+  }
+
   var opts = styleSettings.checkStyleSettings(filePath, textEditor)
 
   opts.cwd = path.dirname(filePath)