[DOP-9787] Test strategies when DBReader reads empty source

[dolfinus]

Change Summary

If DBReader reads data from source (including empty source), the resulting dataframe contains all the data from the source at the moment of calling .run(). But some data is appended to the source after .run() is called, the result if different for different strategies:

1. Dataframe returned within SnapshotStrategy always contains all the source data. This is because no additional filters are applied to original query, and Spark dataframes are lazy.
2. Other strategies return dataframes which contains only data present in the source at the moment of calling .run(), but not data added to source after it. This is because filters based on current HWM value & strategy are baked into the query.

Added tests covering these edge cases.

Handling case source is empty -> HWM value is None -> cannot apply filter to query -> return e empty dataframe required adding new limit option to DBConnection.read_source_as_df. Passing limit=0 returns dataframe which is always empty, which is implementation detail.

Also improved behavior of batch strategies - if source contains no data, instead of raising ValueError just stop after first iteration, and return empty dataframe.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (2b03409) 94.19% compared to head (f8df474) 94.30%.

Files	Patch %	Lines
onetl/strategy/batch_hwm_strategy.py	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #188      +/-   ##
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+ Coverage    94.19%   94.30%   +0.10%     
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  Files          204      204              
  Lines         7684     7705      +21     
  Branches      1347     1355       +8     
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+ Hits          7238     7266      +28     
+ Misses         330      326       -4     
+ Partials       116      113       -3     

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