Don't warn when there are no source files.
Don't warn when there are no source files.

This is an expected result, e.g., when using wildcards, or something like
https://github.com/tschaub/grunt-newer that only removes unchanged files from
the source list.

yoniholmes/grunt-text-replace,seanzarrin/grunt-text-replace,micahblu/grunt-replace-text