Fix overly strict function validation requirements for decoding parameters

[gnidan]

This should fix #1916 and #2065.
Arises from trufflesuite/truffle#1487. Also being tracked on Truffle's end at trufflesuite/truffle#1504.

There is a bit of history here, in that empty bytes arrays have been variously represented as:

• "0x" (correct value, others malformed)
• "0X"
• "0x0"
• ""
• who knows what else

Anyway, Ganache was passing "0x0" for awhile, and now that's stopped, exposing this conditional that asserts bytes must contain data. In the case where outputs equals [], an empty value for bytes is really all that makes sense.

This PR thus loosens the conditional a bit, to throw the exception only if bytes is empty and outputs is non-empty.

[nivida]

I reference here the PR #2000 just to be sure this fix will not be overwritten on merging.

[coveralls]

Coverage remained the same at 83.071% when pulling 617fd0b on trufflesuite:fix-mishandledBytes into 613a222 on ethereum:1.0.