Properly handle null field values when encoding requests.

[bobjackman]

This fixes Issue #75

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[kevmoo]

Create a local variable and do this in two steps. It'll be easier to read.

Also: please add a test that exercises this case.

[bobjackman]

Both done. I don't have sdk v1.23 installed due to local requirements for 1.22, so I couldn't actually run the test suite, but the tests were super simple changes, so I don't expect any problems there.

[googlebot]

CLAs look good, thanks!

[donny-dont]

@kevmoo I don't see any references to mapToQuery in master. Shouldn't this target the 0.11 branch?

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[bobjackman]

@donny-dont you're totally right, my bad. I just pushed a rebase and changed the PR base.

Also, not sure what happened to my cla... it was there...

[nex3]

See #75 (comment)