Return nil proxy function if proxy not enabled

[zeripath]

Unfortunately #16704 contains a bug whereby if the proxy is not enabled all requests fail due to a panic.

This PR sets the proxy function to return a nil function if the proxy is not enabled.

Signed-off-by: Andrew Thornton art27@cantab.net

[zeripath]

Please note:

As I mentioned #16704 (comment) #16704 is BREAKING as Gitea will now only use a system defined proxy if you actively turn on the proxy service.

[codecov-commenter]

Codecov Report

Merging #16742 (e3149c7) into main (4debb74) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main   #16742   +/-   ##
=======================================
  Coverage   45.37%   45.37%           
=======================================
  Files         760      760           
  Lines       85482    85483    +1     
=======================================
+ Hits        38785    38791    +6     
+ Misses      40414    40412    -2     
+ Partials     6283     6280    -3     

Impacted Files	Coverage Δ
modules/proxy/proxy.go	15.15% <0.00%> (-3.60%)	⬇️
modules/process/manager.go	72.83% <0.00%> (-2.47%)	⬇️
modules/log/event.go	60.64% <0.00%> (-1.86%)	⬇️
models/repo_list.go	77.64% <0.00%> (-0.79%)	⬇️
services/pull/pull.go	42.19% <0.00%> (+0.40%)	⬆️
routers/api/v1/repo/pull.go	29.63% <0.00%> (+0.51%)	⬆️
modules/queue/workerpool.go	55.72% <0.00%> (+1.14%)	⬆️
modules/queue/unique_queue_disk_channel.go	48.63% <0.00%> (+1.36%)	⬆️
modules/queue/manager.go	68.18% <0.00%> (+2.84%)	⬆️

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