Allow generators which don't populate optional fields about protofuzz

[dguido]

Thanks! We'll take a look at this soon.

[csboling]

My patch is definitely functionally incorrect, issues I have discovered include:

• Some messages never produce more than a few permutations, e.g. one with and then one without a submessage
• Permutations which omit some submessages seem not to show up for >100,000 permutations in some cases, and then to come one after another

I'm not sure exactly what the issue is but the way I did it in the linked patch was kind of last-ditch. I wanted to use the Product combinator to make "don't descend into this submessage" a possibility but it seemed as though some of the code was built around the assumption that "values" to fuzz should be leaf fields/primitive types, not compound Protobuf message fields.