server: Don't fail to get Transcode Results if Detections header missing

[thomshutt]

What does this pull request do? Explain your changes. (required)
The "Scene Detection" feature should currently be a no-op unless explicitly enabled with a flag, but the current implementation fails to return Transcode results for a standalone T if detection isn't enabled.

Specific updates (required)

• Only try to parse the Detections header if it's present
• Only send the Detections header if detections have been performed

How did you test each of these updates (required)

• Added unit tests
• Ran O / T locally

Does this pull request close any open issues?
fix #2459

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[codecov]

Codecov Report

Merging #2465 (c832aec) into master (530051b) will increase coverage by 0.14625%.
The diff coverage is 10.00000%.

❗ Current head c832aec differs from pull request most recent head 0216d99. Consider uploading reports for the commit 0216d99 to get more accurate results

@@                 Coverage Diff                 @@
##              master       #2465         +/-   ##
===================================================
+ Coverage   54.86667%   55.01292%   +0.14625%     
===================================================
  Files             94          94                 
  Lines          19726       19739         +13     
===================================================
+ Hits           10823       10859         +36     
+ Misses          8306        8278         -28     
- Partials         597         602          +5     

Impacted Files	Coverage Δ
cmd/livepeer_cli/livepeer_cli.go	0.00000% <0.00000%> (ø)
cmd/livepeer_cli/wizard_transcoder.go	0.00000% <0.00000%> (ø)
server/ot_rpc.go	35.75581% <5.26316%> (+10.53586%)	⬆️
server/handlers.go	53.51598% <25.00000%> (-0.10456%)	⬇️
server/webserver.go	95.91837% <100.00000%> (+0.04207%)	⬆️
discovery/db_discovery.go	67.01389% <0.00000%> (-1.04167%)	⬇️

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