Make sure flagValue.String() can be called even when flagValue is created by reflect.New

[ogaca-dd]

Description

Make sure flagValue.String() can be called even when flagValue is created by reflect.New

import (
	"flag"
	"log"

	"go.opentelemetry.io/collector/featuregate"
)

func main() {
	fs := new(flag.FlagSet)

	featuregate.GlobalRegistry().RegisterFlags(fs)
	if err := fs.Parse([]string{"--help"}); err != nil {
		log.Fatalf("failed to parse flags: %v", err)
	}
}

Before this PR

Usage:
  -feature-gates value
        Comma-delimited list of feature gate identifiers. Prefix with '-' to disable the feature. '+' or no prefix will enable the feature.

panic calling String method on zero featuregate.flagValue for flag feature-gates: runtime error: invalid memory address or nil pointer dereference
failed to parse flags: flag: help requested

Note that there is a panic in the message.

After this PR

Usage:
  -feature-gates value
        Comma-delimited list of feature gate identifiers. Prefix with '-' to disable the feature. '+' or no prefix will enable the feature.
failed to parse flags: flag: help requested

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ogaca-dd / name: Olivier G (70e1101, 2a97dfc, 2e90fe5)
• ✅ login: songy23 / name: Yang Song (75f83c3)

[songy23]

LGTM, does this need a changelog?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.64%. Comparing base (9c4c007) to head (2a97dfc).
Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11651      +/-   ##
==========================================
+ Coverage   91.58%   91.64%   +0.05%     
==========================================
  Files         440      442       +2     
  Lines       23765    23751      -14     
==========================================
  Hits        21766    21766              
+ Misses       1627     1616      -11     
+ Partials      372      369       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests