Avoid potential NPE in isIdentifierPart.

If the VirtualFile instance is null, isIdentifierPart attempts to
compare fileType to the Ruby file type identifier, but the way the
string comparison is executed will trigger an NPE instead of just having
isIdentifierPart return false.
Merge branch 'filetype-fixes' of https://github.com/agatti/IntelliJDashPlugin into agatti-filetype-fixes
fixing fileType issue