BULK MISC FIXES: Add error counters; Fix null string handlings

[Deep1998]

Add error counters for reader and transformer.
Fix null string handlings

[codecov]

Codecov Report

Attention: Patch coverage is 93.10345% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 40.27%. Comparing base (5597215) to head (a97250a).

❗ Current head a97250a differs from pull request most recent head 3030f70. Consider uploading reports for the commit 3030f70 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1533      +/-   ##
============================================
+ Coverage     40.18%   40.27%   +0.08%     
- Complexity     2791     2813      +22     
============================================
  Files           737      741       +4     
  Lines         42768    42914     +146     
  Branches       4578     4603      +25     
============================================
+ Hits          17186    17282      +96     
- Misses        24100    24140      +40     
- Partials       1482     1492      +10     

Components	Coverage Δ
spanner-templates	56.84% <93.10%> (+0.05%)	⬆️
spanner-import-export	65.52% <ø> (+0.02%)	⬆️
spanner-live-forward-migration	63.14% <92.85%> (-0.03%)	⬇️
spanner-live-reverse-replication	42.94% <92.85%> (-0.02%)	⬇️
spanner-bulk-migration	70.05% <93.10%> (+0.08%)	⬆️

Files	Coverage Δ
...jdbc/dialectadapter/mysql/MysqlDialectAdapter.java	100.00% <100.00%> (ø)
.../reader/io/jdbc/rowmapper/JdbcSourceRowMapper.java	100.00% <100.00%> (ø)
...leport/v2/transformer/SourceRowToMutationDoFn.java	100.00% <100.00%> (ø)
...er/migrations/avro/GenericRecordTypeConvertor.java	95.20% <100.00%> (-0.12%)	⬇️
...le/cloud/teleport/v2/constants/MetricCounters.java	0.00% <0.00%> (ø)
.../v2/spanner/migrations/avro/AvroToValueMapper.java	92.46% <88.88%> (-0.50%)	⬇️

... and 38 files with indirect coverage changes

[bharadwaj-aditya]

LGTM

[bharadwaj-aditya]

LGTM