Default signature for new identities only

app/src/main/java/eu/faircode/email/FragmentIdentity.java

@@ -849,7 +849,7 @@ public class FragmentIdentity extends FragmentBase {
                     etDisplay.setText(identity == null ? null : identity.display);
 
                     String signature = (identity == null ? null : identity.signature);
-                    if (TextUtils.isEmpty(signature)) {
+                    if (identity == null) {
                         CharSequence promote = getText(R.string.app_promote);
                         if (promote instanceof Spanned)
                             signature = HtmlHelper.toHtml((Spanned) promote);