Reject None for non-optional field in coerce mode

[AllanChain]

Currently, type_check=coerce lets the deserializer silently accept None for a non-optional field, which is inconsistent with the documentation. For example:

import serde

@serde.serde(type_check=serde.coerce)
class A:
    a: int = 0

print(serde.from_dict(A, {"a": None}))

The above code outputs the following before the fix:

A(a=None)

But a SerdeError is expected.

Here, we raise SerdeError when coerce_object() receives None for a non-optional field.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.90%. Comparing base (f09d503) to head (387961c).
⚠️ Report is 69 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #746      +/-   ##
==========================================
+ Coverage   90.83%   90.90%   +0.06%     
==========================================
  Files          13       13              
  Lines        2040     2308     +268     
  Branches      370      451      +81     
==========================================
+ Hits         1853     2098     +245     
- Misses        124      140      +16     
- Partials       63       70       +7     

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