add null check for text in TextLayoutBackend

the text should never be null, because the toolkits require
a string value, when drawing. Otherwise WPF and MAC
throw a NullReferenceException and Gtk prints
a warning to the console (Pango-CRITICAL).
Merge pull request #382 from sevoku/fix-textlayoutbackend-null-string

add null check for text in TextLayoutBackend