Fix GodotFetch glue code for null response bodies

The spec says that Response.body can be null (in the event of requests that should have no body, like HEAD requests) and Firefox adheres to it which results in request failure for HEAD requests on Firefox for web exports.

This commit addresses that by treating a null body as an "empty" body (without using a polyfill) and avoids changing the request lifecycle as much as possible.

PR review changes:
- Use == instead of strict ===
- Do not use ?? null
- Comment formatting

platform/web/js/libs/library_godot_fetch.js

@@ -59,7 +59,12 @@ const GodotFetch = {
 			});
 			obj.status = response.status;
 			obj.response = response;
-			obj.reader = response.body.getReader();
+			// `body` can be null per spec (for example, in cases where the request method is HEAD).
+			// As of the time of writing, Chromium (127.0.6533.72) does not follow the spec but Firefox (131.0.3) does.
+			// See godotengine/godot#76825 for more information.
+			// See Chromium revert (of the change to follow the spec):
+			// https://chromium.googlesource.com/chromium/src/+/135354b7bdb554cd03c913af7c90aceead03c4d4
+			obj.reader = response.body?.getReader();
 			obj.chunked = chunked;
 		},
 
@@ -121,6 +126,10 @@ const GodotFetch = {
 				}
 				obj.reading = true;
 				obj.reader.read().then(GodotFetch.onread.bind(null, id)).catch(GodotFetch.onerror.bind(null, id));
+			} else if (obj.reader == null && obj.response.body == null) {
+				// Emulate a stream closure to maintain the request lifecycle.
+				obj.reading = true;
+				GodotFetch.onread(id, { value: undefined, done: true });
 			}
 		},
 	},
@@ -159,7 +168,10 @@ const GodotFetch = {
 		if (!obj.response) {
 			return 0;
 		}
-		if (obj.reader) {
+		// If the reader is nullish, but there is no body, and the request is not marked as done,
+		// the same status should be returned as though the request is currently being read
+		// so that the proper lifecycle closure can be handled in `read()`.
+		if (obj.reader || (obj.response.body == null && !obj.done)) {
 			return 1;
 		}
 		if (obj.done) {