Fix decode issue for local activities returning nil result (#943)

[vytautas-karpavicius]

What changed?
Fix decode issue for local activities returning nil result (#943)

Why?
When local activity returns nil result, it is encoded into nil byte
slice ([]byte). Later this is converted into string field ResultJSON
within localActivityMarkerData structure. Such casting results in
empty string, as this is its zero value in golang.

Reverse conversion is not simetrical. Casting empty string back to
byte slice results in empty byte slice. This later causes EOF error
when trying to decode it back to the original result.

The fix is to check whether it is non-empty string and only then
assign the result back. This will be simetrical and will not cause
decoding error later.

How did you test it?
This change also adds additional replay tests, that covers this
scenario among many other local activity related cases.

Potential risks

[coveralls]

Pull Request Test Coverage Report for Build fc7169fc-8cd2-433e-9127-2d2019690935

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 74.675%

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Totals
Change from base Build 194eea71-73fe-406a-a5fd-4d250fc1366e:	0.04%
Covered Lines:	9359
Relevant Lines:	12533

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💛 - Coveralls

[vancexu]

Another related task when you have time, could you also check if there are similar edge cases in other marker events (SideEffect/MutableSideEffect/GetVersion)? if not, maybe worth to add unit tests as what you provide in this PR. Thanks

[meiliang86]

Fixes #943