Fix false duplicate source code path error for git-sourced apps

[denik]

Changes

• Fix false "Duplicate app source code path" error when multiple apps use git_source instead of source_code_path. The duplicate check keyed on SourceCodePath, so all git-sourced apps collided on the empty string. Now skips the check when SourceCodePath is empty.

Why

Users with multiple git-sourced apps in a bundle get a spurious validation error that blocks bundle validate and bundle deploy.

Tests

• Acceptance test with two git-sourced apps verifies validation passes without false duplicate error.
• Confirmed the test fails on v0.296.0 (latest release) as a regression check.

Output of go test ./acceptance -run TestAccept/bundle/apps/git_source_no_duplicate -useversion 0.296.0:

[CLI] bundle validate
Error: Duplicate app source code path

app resource 'app1' has the same source code path as app resource 'app2', this will lead to the app configuration being overriden by each other

[github-actions]

Approval status: pending

/acceptance/bundle/ - needs approval

Files: acceptance/bundle/apps/git_source_no_duplicate/databricks.yml, acceptance/bundle/apps/git_source_no_duplicate/out.test.toml, acceptance/bundle/apps/git_source_no_duplicate/output.txt, acceptance/bundle/apps/git_source_no_duplicate/script, acceptance/bundle/apps/git_source_no_duplicate/test.toml
Suggested: @andrewnester
Also eligible: @pietern, @lennartkats-db, @shreyas-goenka, @anton-107, @janniklasrose

/bundle/ - needs approval

Files: bundle/apps/validate.go
Suggested: @andrewnester
Also eligible: @pietern, @lennartkats-db, @shreyas-goenka, @anton-107, @janniklasrose

General files (require maintainer)

Files: NEXT_CHANGELOG.md
Based on git history:

• @andrewnester -- recent work in ./, bundle/apps/

_{Any maintainer (@andrewnester, @anton-107, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db) can approve all areas.
See OWNERS for ownership rules.}