service: Handle zero-value in stringPtrFlag.String

FlagSet.PrintDefaults uses reflection to construct a zero value, calls
.String on it, and compares the result with the current flag value to
detect zero-value flags. For stringPtrFlag, this would result in a
panic, as String() always dereferenced the first level of its **string.

Add another check so that both pointer levels are nil-checked.

service.go

@@ -250,7 +250,7 @@ type stringPtrFlag struct {
 }
 
 func (f stringPtrFlag) String() string {
-	if *f.ptr == nil {
+	if f.ptr == nil || *f.ptr == nil {
 		return ""
 	}
 	return **f.ptr