Disable call-to-action button when hostname is empty (#1538)

I noticed a tiny UI glitch, where we are inconsistent with our style
guide: [call-to-action buttons should be disabled, if the corresponding
input field is
empty](https://github.com/tiny-pilot/tinypilot/blob/1354e914f364c5e1de5d922ea0a68fc7e976f3df/app/templates/styleguide.html#L168-L171).
This PR fixes this issue for the hostname dialog.


https://github.com/tiny-pilot/tinypilot/assets/83721279/a06eeeea-5bb2-45f5-8f48-9ac2aa639ff9


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app/templates/custom-elements/change-hostname-dialog.html

@@ -217,7 +217,9 @@ <h3>Changing Hostname</h3>
         _onInputChanged() {
           const isEqualToInitialValue =
             this.elements.hostnameInput.value === this.initialHostname;
-          this.elements.changeAndRestart.disabled = isEqualToInitialValue;
+          const isEmpty = this.elements.hostnameInput.value === "";
+          this.elements.changeAndRestart.disabled =
+            isEmpty || isEqualToInitialValue;
         }
 
         _doChangeHostname() {