fix: allow empty string to reset StringToString flag

[happysnaker]

Summary

Fixes #312

Currently, passing an empty string to a StringToString flag (e.g., --flag="") returns an error because the parser expects a key=value format. This makes it impossible to override a non-empty default map with an empty one.

Changes

Allow an empty string value to reset the flag to an empty map. When val == "", the existing map entries are cleared and changed is set to true.

Before

fs := pflag.NewFlagSet("test", pflag.ExitOnError)
fs.StringToString("x", map[string]string{"a":"1"}, "")
fs.Set("x", "")  // Error: must be formatted as key=value

After

fs.Set("x", "")  // OK: map is now empty