Fix issues for sequences and identity columns

[hengfengli]

Fix a few issues related to sequences and identity columns:

• null strings appear in identity columns. If a parameter is null, it should not appear in the output file.
• A conversion is required from BIT_REVERSED_POSITIVE_SEQUENCE to bit_reversed_positive because the identity_kind in the information_schema table has a value BIT_REVERSED_POSITIVE_SEQUENCE which is different than bit_reversed_positive.
• sequence_kind="default" is not shown properly for the statement CREATE SEQUENCE myseq.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.06%. Comparing base (c0cadc4) to head (beb1571).
Report is 16 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2171      +/-   ##
============================================
+ Coverage     46.95%   47.06%   +0.10%     
- Complexity     4044     4053       +9     
============================================
  Files           876      876              
  Lines         52190    52226      +36     
  Branches       5501     5508       +7     
============================================
+ Hits          24507    24580      +73     
+ Misses        25924    25887      -37     
  Partials       1759     1759              

Components	Coverage Δ
spanner-templates	68.92% <100.00%> (+0.04%)	⬆️
spanner-import-export	65.75% <100.00%> (+0.05%)	⬆️
spanner-live-forward-migration	76.48% <ø> (-0.03%)	⬇️
spanner-live-reverse-replication	78.80% <ø> (+0.12%)	⬆️
spanner-bulk-migration	87.94% <ø> (+<0.01%)	⬆️

Files with missing lines	Coverage Δ
...oud/teleport/spanner/DdlToAvroSchemaConverter.java	93.12% <100.00%> (+0.07%)	⬆️

... and 14 files with indirect coverage changes

[manitgupta]

@darshan-sj Do we also test this functionality in an IT? Do we need to update a corresponding IT as well?

[darshan-sj]

Can you check ExportPipelineIT and ImportPipelineIT and please add a test case if it is possible to add for this edgecase.

[darshan-sj]

LGTM