Don't use database as fallback when no schema parsed.

[JDarDagran]

Problem

With cross-database queries to information schema there shouldn't be used database as fallback for no schema parsed.

Solution

Set schema to None in TablesHierarchy. This will lead to no filter on schema level in information schema query.

Checklist

• You've signed-off your work
• Your pull request title follows our guidelines
• Your changes are accompanied by tests (if relevant)
• Your change contains a small diff and is self-contained
• You've updated any relevant documentation (if relevant)
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SPDX-License-Identifier: Apache-2.0
Copyright 2018-2023 contributors to the OpenLineage project

[codecov-commenter]

Codecov Report

Merging #2023 (93e71e0) into main (6b95181) will increase coverage by 1.36%.
The diff coverage is n/a.

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@@             Coverage Diff              @@
##               main    #2023      +/-   ##
============================================
+ Coverage     81.14%   82.51%   +1.36%     
============================================
  Files            88       54      -34     
  Lines          3724     3140     -584     
  Branches         29        0      -29     
============================================
- Hits           3022     2591     -431     
+ Misses          666      549     -117     
+ Partials         36        0      -36     

Files Changed	Coverage Δ
...ow/openlineage/airflow/extractors/sql_extractor.py	90.37% <ø> (ø)

... and 34 files with indirect coverage changes

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