Proxy: do not store empty key

[khanova]

Problem

Currently we store in cache even if the project is undefined. That makes invalidation impossible.

Summary of changes

Do not store if project id is empty.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2262 tests run: 2179 passed, 0 failed, 83 skipped (full report)

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Flaky tests (4)

Postgres 15

• test_statvfs_pressure_usage: debug

Postgres 14

• test_lfc_resize: debug
• test_pg_regress: debug
• test_empty_tenant_size: debug

Code coverage (full report)

• functions: 54.5% (10601 of 19448 functions)
• lines: 81.6% (60683 of 74406 lines)

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_{The comment gets automatically updated with the latest test results
10bf61c at 2024-01-20T16:25:29.922Z :recycle:}