add empty to context flags

[MichelleArk]

resolves #10317
resolves #10152

Problem

We are missing the --empty flag value from the jinja context flags, making it tricky to overwrite the ref macro in a way that incorporates the value of --empty

Solution

Add empty to jinja flags + unit test

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

🎩
(adding {{ log(flags.EMPTY, info=True) }} to locations.sql)

❯ dbt run --empty  --select locations
...
14:42:08  1 of 1 START sql view model test_arky.locations ................................ [RUN]
14:42:08  True

❯ dbt run --select locations
...
14:42:08  1 of 1 START sql view model test_arky.locations ................................ [RUN]
14:42:08  False

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.71%. Comparing base (27b2f96) to head (ace8aca).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10315      +/-   ##
==========================================
- Coverage   88.76%   88.71%   -0.05%     
==========================================
  Files         180      180              
  Lines       22488    22488              
==========================================
- Hits        19962    19951      -11     
- Misses       2526     2537      +11     

Flag	Coverage Δ
integration	85.97% <ø> (-0.13%)	⬇️
unit	61.84% <ø> (ø)

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