fix(rds): add comprobations before list tags

[puchy22]

Context

If Event Subscription does not have ARN not try to list tags.

Description

Added attributes check before listing tags.

Checklist

• Are there new checks included in this PR? No.
  • If so, do we need to update permissions for the provider? No.
• Review if the code is being covered by tests.
• Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• Review if backport is needed.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sergargar]

👏🏼

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.10%. Comparing base (8805230) to head (f742d65).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5249      +/-   ##
==========================================
- Coverage   89.12%   89.10%   -0.02%     
==========================================
  Files         993      993              
  Lines       30509    30510       +1     
==========================================
- Hits        27190    27187       -3     
- Misses       3319     3323       +4     

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