(bug 42960) extension loader now ignores empty directories

The /tools/extensions-loader.php is injected in LocalSettings.php for
mwext- jobs. It attempts to load a default entry point for each
directory under $WORKSPACE/extensions.

When submodules are not properly initialized, the directory is empty. We
thus skip loading any empty extension.

Change-Id: I36b6e1dee68f039b7c08214d5b8e6838d42cdc60