Batching - allow null entries

[igorbernstein2]

This came out of working on googleapis/google-cloud-java#3026.

A Bigtable MutateRowsResponse contains a list of entries. Each entry simply contains a Status indicating if that entry succeeded or failed. Since the user only cares if the entry failed, the most useful type signature for a BatchingCallable representing the RPC would be BatchingCallable<MutateRowsRequest, Void>.

Unfortunately, BatchedRequestIssuer treats null response as absent. This PR adds an explicit hasResponse flag to check for missing responses, which allows for null responses for Void entries

[codecov-io]

Codecov Report

Merging #496 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #496 +/- ##
===========================================
+ Coverage 70.5% 70.5% +<.01% 
Complexity 810 810 ===========================================
Files 165 165 Lines 3773 3774 +1 Branches 289 289 ===========================================
+ Hits 2660 2661 +1 
Misses 990 990 Partials 123 123

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/api/gax/rpc/BatchedRequestIssuer.java	100% <100%> (ø)	9 <5> (ø)	⬇️

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[garrettjonesgoogle]

LGTM