Warn about blank usernames

src/main/java/com/cloudbees/plugins/credentials/impl/UsernamePasswordCredentialsImpl.java

@@ -146,5 +146,18 @@
             }
             return FormValidation.ok();
         }
+
+        @RequirePOST
+        public FormValidation doCheckUsername(@QueryParameter String value, @QueryParameter boolean usernameSecret) {
+            if (value.isEmpty()) {
+                if (usernameSecret) {
+                    return FormValidation.warning(Messages.UsernamePasswordCredentialsImpl_a_blank_username_cannot_logically_be_con());
+                } else {
+                    return FormValidation.ok(Messages.UsernamePasswordCredentialsImpl_blank_username_did_you_mean_to_use_secre());
+                }
+            } else {
+                return FormValidation.ok();
+            }
+        }
     }
 }

src/main/resources/com/cloudbees/plugins/credentials/impl/Messages.properties

@@ -22,6 +22,8 @@
 # THE SOFTWARE.
 #
 UsernamePasswordCredentialsImpl.DisplayName=Username with password
+UsernamePasswordCredentialsImpl.a_blank_username_cannot_logically_be_con=A blank username cannot logically be considered secret.
+UsernamePasswordCredentialsImpl.blank_username_did_you_mean_to_use_secre=Blank username; did you mean to use secret text credentials instead?
 CertificateCredentialsImpl.DisplayName=Certificate
 CertificateCredentialsImpl.EmptyKeystore=Empty keystore
 CertificateCredentialsImpl.LoadKeyFailed=Couldn''t retrieve key for alias "{0}"