Route53: update_health_check() should handle falsy values correctly

[bblommers]

Fixes #10013

Values should always be updated if they are not None. The user only mentioned the boolean values not being set, but this applies to all falsy values - [], 0 are also valid values.

[bpandola]

Thanks for fixing this, @bblommers! These boolean strings have been a real pain during the request parsing/response serialization refactor, but I still think it's important that we convert them. Unfortunately, the three in question here had no test coverage and I failed to spot them. I appreciate you jumping on this so quickly!