fix: improve coherency and robusteness of CsvDataReader

[Seddryck]

• this[] returns the same value than GetValue
• GetFieldValue also works when no schema is defined.
• FieldType returns object when no schema is defined or field is not defined

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.99%. Comparing base (f15eb30) to head (64895e2).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
PocketCsvReader/CsvDataReader.cs	55.55%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
+ Coverage   81.13%   81.99%   +0.86%     
==========================================
  Files          46       46              
  Lines        1362     1355       -7     
  Branches      247      243       -4     
==========================================
+ Hits         1105     1111       +6     
+ Misses        181      173       -8     
+ Partials       76       71       -5     

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