Fix nil pointer in parentRef namespace dereference

[adam-cattermole]

If the parentRef.Namespace does not exist we crash loop trying to locate the gateway namespace. If it does not exist it should be in the namespace of the HTTPRoute.

Fixes #333

[codecov]

Codecov Report

Merging #335 (6caea70) into main (cb9d3d5) will increase coverage by 0.60%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #335      +/-   ##
==========================================
+ Coverage   64.44%   65.05%   +0.60%     
==========================================
  Files          35       35              
  Lines        3794     3794              
==========================================
+ Hits         2445     2468      +23     
+ Misses       1156     1131      -25     
- Partials      193      195       +2     

Flag	Coverage Δ
unit	59.50% <100.00%> (+1.29%)	⬆️

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Components	Coverage Δ
api/v1beta1 (u)	∅ <ø> (∅)
pkg/common (u)	76.92% <100.00%> (+2.99%)	⬆️
pkg/istio (u)	30.24% <ø> (ø)
pkg/log (u)	31.81% <ø> (ø)
pkg/reconcilers (u)	33.68% <ø> (ø)
pkg/rlptools (u)	56.46% <ø> (ø)
controllers (i)	69.94% <ø> (ø)

Files	Coverage Δ
pkg/common/gatewayapi_utils.go	68.23% <100.00%> (+5.41%)	⬆️