Fix NPE if themeContext is null

When installing a new theme it sometimes happens that the
ThemesProvider crashes due to a NullPointerException in
hasThemeComponent(...) when executing
AssetManager assetManager = themeContext.getAssets();

Don't silently catch the NameNotFoundException and return an
empty map.

Change-Id: I9d7bf8f79b9c38cd5da668a7fcf205406ffdd79c
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>