chore: form binding trim space (#38978)

Use "binding:TrimSpace" instead of fragile IsEmptyString

And fix a bug in locale's `HasKey`: it should also try the default
language if current language doesn't have the translation key, a new
test is added.

services/forms/auth_form.go

@@ -87,6 +87,6 @@ type AuthenticationForm struct {
 	SSPIAutoCreateUsers      bool
 	SSPIAutoActivateUsers    bool
 	SSPIStripDomainNames     bool
-	SSPISeparatorReplacement string `binding:"AlphaDashDot;MaxSize(5)"`
+	SSPISeparatorReplacement string `binding:"TrimSpace;AlphaDashDot;MaxSize(5)"`
 	SSPIDefaultLanguage      string
 }