fix: return empty catalog name

[olavloite]

The JDBC connection would return the current database name as the current catalog name, but that catalog name would not be returned in the ResultSet returned by getCatalogs(). This discrepancy causes some tools to ignore or misinterpret the catalog and schema structure of a Cloud Spanner database. Specifically, it breaks the autocomplete feature of DBeaver.

See also dbeaver/dbeaver#9127

[codecov]

Codecov Report

Merging #174 into master will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #174      +/-   ##
============================================
- Coverage     72.20%   72.15%   -0.06%     
+ Complexity     1095     1094       -1     
============================================
  Files            24       24              
  Lines          3292     3282      -10     
  Branches        495      490       -5     
============================================
- Hits           2377     2368       -9     
+ Misses          722      720       -2     
- Partials        193      194       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/cloud/spanner/jdbc/JdbcConnection.java	49.72% <100.00%> (ø)	42.00 <0.00> (ø)
...gle/cloud/spanner/jdbc/AbstractJdbcConnection.java	63.88% <0.00%> (-1.39%)	20.00% <0.00%> (-1.00%)
...ud/spanner/jdbc/AbstractJdbcPreparedStatement.java	90.24% <0.00%> (-0.61%)	52.00% <0.00%> (ø%)
...oogle/cloud/spanner/jdbc/JdbcDatabaseMetaData.java	95.23% <0.00%> (-0.01%)	180.00% <0.00%> (ø%)
...m/google/cloud/spanner/jdbc/JdbcTypeConverter.java	73.91% <0.00%> (+0.11%)	91.00% <0.00%> (ø%)
...a/com/google/cloud/spanner/jdbc/JdbcResultSet.java	79.02% <0.00%> (+0.20%)	186.00% <0.00%> (ø%)

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