Correct uniqueness lookup for nil location

[peterbroadhurst]

It's extremely hard for an external client calling the AddContractListener REST API to determine whether the listener already exists, because the details are a factor of a complex FFI payload.

For this reason FireFly provides a uniqueness check for the:

• namespace
• topic - allows multiple listeners to the same event, for different apps
• location - can be nil, or a blockchain-specific location
• signature - generated by FireFly in a protocol specific way from the supplied FFI signature

However, the code doing a lookup in the DB was not working for the nil check, due to a .ToString()

Annoyingly there's a weirdness with fftypes.JSONAny.Value() in the nil case, and for some reason passing a (*fftypes.JSONAny)(nil) into fb.Eq() is not generating an IS NULL SQL query.

So instead this code passes actual nil in the nil case, and this works.

This is really an underlying issue in firefly-common for a ffapi.JSONField case, but that's too risky a change to do in isolation

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4645f28) 99.99% compared to head (a1c2760) 99.99%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1433   +/-   ##
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  Coverage   99.99%   99.99%           
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  Files         321      321           
  Lines       23092    23095    +3     
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+ Hits        23090    23093    +3     
  Misses          1        1           
  Partials        1        1           

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