xdsclient: typed config nil better nil checks

[wooffie]

We check for nil and in next line deference it tc.TypeUrl?
I found two places, which takes part in it

RELEASE NOTES:

• xds: Fix possible panic when certain invalid resources are encountered

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: wooffie / name: Burkov Egor (12265bb, 17bf5bf, 0f7bd03, 8e947da, 0d30ce4)

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.49%. Comparing base (6207142) to head (17bf5bf).
Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
...ds/internal/xdsclient/xdsresource/unmarshal_cds.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8412      +/-   ##
==========================================
+ Coverage   82.26%   82.49%   +0.22%     
==========================================
  Files         414      414              
  Lines       40424    40434      +10     
==========================================
+ Hits        33255    33355     +100     
+ Misses       5802     5734      -68     
+ Partials     1367     1345      -22     

Files with missing lines	Coverage Δ
xds/internal/xdsclient/xdsresource/filter_chain.go	92.91% <100.00%> (ø)
...ds/internal/xdsclient/xdsresource/unmarshal_cds.go	86.72% <0.00%> (ø)

... and 41 files with indirect coverage changes

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[dfawley]

It looks like there are tests expecting a specific error string:

...
transport_socket missing typed_config or wrong type_url: "type.googleapis.com/envoy.extensions.transport_sockets.tls.v3.UpstreamTlsContext" wantErr: "transport_socket field has unexpected typeURL"

[wooffie]

I've tried to do naive replacement with new error massage, but it broke another tests. I will recheck this =)

[dfawley]

Thank you!