Adding support for default null decimals

[chapm250]

No description provided.

[codecov]

Codecov Report

Merging #130 (a4c92fd) into master (aea206b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #130   +/-   ##
=======================================
  Coverage   99.61%   99.62%           
=======================================
  Files           9        9           
  Lines         788      792    +4     
  Branches      121      123    +2     
=======================================
+ Hits          785      789    +4     
  Misses          2        2           
  Partials        1        1           

Impacted Files	Coverage Δ
dataclasses_avroschema/fields.py	99.55% <100.00%> (+<0.01%)	⬆️

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[marcosschroh]

Thanks @chapm250 for the work. We can go with this solution but it is not ideal because it won't work with unions:

class MyAvro(AvroModel):
    an_union: typing.Union[int, types.Decimal]

We need to follow a different approach, like we are trying to do with #108