Empty featuregate values will happen to be a panic

[matianjun1]

Description:
Fixing a bug - Empty featuregate values will happen to be a panic

FlagValue.Set("")

panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
go.opentelemetry.io/collector/service/featuregate.FlagValue.setSlice(...)
	/xxx/go/pkg/mod/go.opentelemetry.io/collector@v0.55.0/service/featuregate/flags.go:54
go.opentelemetry.io/collector/service/featuregate.FlagValue.Set(0x105f26828?, {0x0?, 0x0?})
	/xxx/go/pkg/mod/go.opentelemetry.io/collector@v0.55.0/service/featuregate/flags.go:47 +0x14c

Testing:

Add an empty flag test

Documentation:

Just check if the input string is empty, and return directly.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: matianjun1 / name: Chinese.马天骏 (8632ba6)

[codecov]

Codecov Report

Merging #5663 (0733778) into main (58e66af) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5663   +/-   ##
=======================================
  Coverage   91.41%   91.42%           
=======================================
  Files         191      191           
  Lines       11386    11389    +3     
=======================================
+ Hits        10409    10412    +3     
  Misses        778      778           
  Partials      199      199           

Impacted Files	Coverage Δ
service/featuregate/flags.go	100.00% <100.00%> (ø)

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[mx-psi]

👋 thanks for the PR, could you add a changelog entry?

[TylerHelmuth]

LGTM