Fix NPE in RetryParameters and add unit tests to ExecuteActivityParameters

[timl3136]

What changed?
Fix NPE in RetryParameters and add unit tests to ExecuteActivityParameters

Why?
Fix an edge case where set function could result in NPE
Improve unit test coverage

How did you test it?
Unit tests

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.61%. Comparing base (aa86d2a) to head (c487f8b).
Report is 9 commits behind head on master.

Additional details and impacted files

Files with missing lines	Coverage Δ
.../uber/cadence/internal/common/RetryParameters.java	100.00% <100.00%> (+15.38%)	⬆️

... and 7 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 2624

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 175 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.7%) to 78.371%

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Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/uber/cadence/internal/testservice/TestWorkflowMutableStateImpl.java	1	85.56%
src/main/java/com/uber/cadence/internal/logging/ReplayAwareLogger.java	174	6.95%

Totals
Change from base Build 2617:	-0.7%
Covered Lines:	15197
Relevant Lines:	19391

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💛 - Coveralls