fix: allow refresh-token auth without userName

[KaviarasuSakthivadivel]

• DataCloudConnection.of() accepts null userName (optional for refresh-token auth)
• DataCloudDatabaseMetadata.getUserName() throws SQLException with state 28000 when userName is null
• DataCloudDatasource normalizes null userName to empty string for connect path

Fixes regression where driver required userName for refresh-token auth (e.g. 'userName is marked non-null but is null').

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.73%. Comparing base (cf3209d) to head (37683f1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...salesforce/datacloud/jdbc/DataCloudDatasource.java	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #157      +/-   ##
============================================
+ Coverage     82.70%   82.73%   +0.03%     
- Complexity     1345     1347       +2     
============================================
  Files           111      111              
  Lines          4053     4055       +2     
  Branches        413      413              
============================================
+ Hits           3352     3355       +3     
  Misses          510      510              
+ Partials        191      190       -1     

Components	Coverage Δ
JDBC Core	83.91% <100.00%> (+0.04%)	⬆️
JDBC Main	40.69% <0.00%> (ø)
JDBC HTTP	91.09% <ø> (ø)
JDBC Utilities	66.07% <ø> (ø)
Spark Datasource	∅ <ø> (∅)

Files with missing lines	Coverage Δ
...force/datacloud/jdbc/core/DataCloudConnection.java	58.86% <ø> (+0.41%)	⬆️
...datacloud/jdbc/core/DataCloudDatabaseMetadata.java	98.34% <100.00%> (+0.02%)	⬆️
...salesforce/datacloud/jdbc/DataCloudDatasource.java	40.35% <0.00%> (ø)

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