catalog: simplify function isValidTrafficTarget()

[draychev]

This PR simplifies isValidTrafficTarget()

Added a test to cover non-nil but empty Rules.

---

Affected area:

• New Functionality [ ]
• Documentation [ ]
• Install [ ]
• Control Plane [ ]
• CLI Tool [ ]
• Certificate Management [ ]
• Networking [ ]
• Metrics [ ]
• SMI Policy [ ]
• Security [ ]
• Tests [ ]
• CI System [ ]
• Performance [ ]
• Other [ ]

Please answer the following questions with yes/no.

• Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution?

[codecov-io]

Codecov Report

Merging #2246 (911f36f) into main (c48acb9) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2246      +/-   ##
==========================================
+ Coverage   58.17%   58.39%   +0.22%     
==========================================
  Files         146      146              
  Lines        6388     6386       -2     
==========================================
+ Hits         3716     3729      +13     
+ Misses       2667     2652      -15     
  Partials        5        5              

Impacted Files	Coverage Δ
pkg/catalog/routes.go	79.03% <100.00%> (-0.15%)	⬇️
pkg/envoy/route/config.go	96.03% <0.00%> (+0.79%)	⬆️
pkg/configurator/validating_webhook.go	78.98% <0.00%> (+10.14%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c48acb9...911f36f. Read the comment docs.