response none when prosody has no text

[markmacau]

Thanks for great tool. I found that ssml-builder return null when no text such as '' provided for prosody.

This returns ssml properly:
speech.prosody({rate: "110%"},"abc")

This returns null:
speech.prosody({rate: "110%"},"")

Because I use ssml-builder inside for-loop, above situation happened. I solved my problem to check value existence before fire ssml-builder.

I think it is better if there is some error message in such case.

[RokasVaitkevicius]

There is a test in place testing just that and it passes. It throws an error "The word provided to Speech#prosody(..) was empty". The same happens if you try to add empty word in the loop.