fix: handle PgDataType.voidType

[osaxma]

Partly fixes #125

• For PgDataType.voidType, return null.
• For any unimplemented type, return whatever was decoded as text
  • I believe it's better to return something than throw an exception

---

Context:

I was running this query and I received the following error:

--> data ReadyForQueryMessage(state = I)
<-- data Instance of 'QueryMessage'
      sending Qnselect pg_drop_replication_slot('a_test_slot') from pg_replication_slots where slot_name = 'a_test_slot';
--> data Instance of 'RowDescriptionMessage'
--> data Data Row Message: [[]]
Unhandled exception:
UnimplementedError: Text decoding for PgDataType.voidType

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (e22e648) 87.28% compared to head (5805557) 87.21%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
- Coverage   87.28%   87.21%   -0.07%     
==========================================
  Files          29       29              
  Lines        2500     2502       +2     
==========================================
  Hits         2182     2182              
- Misses        318      320       +2     

Files	Coverage Δ
lib/src/text_codec.dart	92.24% <0.00%> (-1.62%)	⬇️

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[isoos]

Maybe?

if (asText is T) return asText;
throw UnimplementedError('Text decoding for $_dataType');

[osaxma]

        if (asText is T) {
          return asText as T; // i had to do it like this; otherwise it gives an error 
        }
        throw UnimplementedError('Text decoding for $_dataType');

[isoos]

thanks!