editable-text: Don't put 'null'/'undefined' in the field

In case something deleted dataset.initialValue after focus and before we
received this keypress.

Addresses #3725. Might still want to rework that listener, but this
change made sense regardless, since the same kind of race condition
could be triggered elsewhere.

chrome/content/zotero/elements/editableText.js

@@ -239,7 +239,7 @@
 					}
 					else if (event.key === 'Escape') {
 						this.dispatchEvent(new CustomEvent('escape_enter'));
-						let initialValue = this._input.dataset.initialValue;
+						let initialValue = this._input.dataset.initialValue ?? '';
 						this.setAttribute('value', initialValue);
 						this._input.value = initialValue;
 						this._input.blur();