Make sure flagValue.String() can be called even when flagValue is created by reflect.New (#11651)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

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

```go
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
```

---------

Co-authored-by: Yang Song <songy23@users.noreply.github.com>

.chloggen/feature-gate-help-cmd-option.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: featuregate
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix an unfriendly display message `runtime error` when featuregate is used to display command line usage.
+
+# One or more tracking issues or pull requests related to the change
+issues: [11651]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

featuregate/flag.go

@@ -31,6 +31,12 @@ type flagValue struct {
 }
 
 func (f *flagValue) String() string {
+	// This function can be called by isZeroValue https://github.com/golang/go/blob/go1.23.3/src/flag/flag.go#L630
+	// which creates an instance of flagValue using reflect.New. In this case, the field `reg` is nil.
+	if f.reg == nil {
+		return ""
+	}
+
 	var ids []string
 	f.reg.VisitAll(func(g *Gate) {
 		id := g.ID()

featuregate/flag_test.go

@@ -132,3 +132,8 @@ func TestNewFlag(t *testing.T) {
 		})
 	}
 }
+
+func TestFlagStringNotInitialize(t *testing.T) {
+	flag := &flagValue{}
+	assert.Empty(t, flag.String())
+}