Fix subscription match nil format.

[Taction]

Description

Metadata API returns rule match value to be empty when it is not defined.

Issue reference

Please reference the issue this PR will close: #7049

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]
• Specification has been updated / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]
• Provided sample for the feature / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]

[ItalyPaleAle]

LGTM overall, please see my comments (and fix the linter!)

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (014d5d8) 64.92% compared to head (56b4f61) 64.96%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7067      +/-   ##
==========================================
+ Coverage   64.92%   64.96%   +0.03%     
==========================================
  Files         231      231              
  Lines       20855    20857       +2     
==========================================
+ Hits        13541    13550       +9     
+ Misses       6182     6167      -15     
- Partials     1132     1140       +8     

Files	Coverage Δ
pkg/grpc/universalapi/api_metadata.go	92.75% <100.00%> (ø)
pkg/expr/expr.go	77.77% <0.00%> (-3.62%)	⬇️

... and 5 files with indirect coverage changes

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[JoshVanL]

@Taction Please can you add an integration test to cover this? Here is an example of an existing metadata test.

[Taction]

@JoshVanL Thanks for the example, added.