fix: an empty or HTML original fails the export

fetch checked archives only, so a System.dat served as an HTML page with 200, or a 0-byte cache, reached render() as a file with no entries. System.dat, es_bios.xml, RetroBat and RomM then rebuilt the platform's file from our data and reported OK. Such an original is now refused, before it is cached and when read from the cache, and a format that needs its original fails when none was read.

tests/test_exporters.py

@@ -1010,6 +1010,28 @@ class CachedOriginal(unittest.TestCase):
         )
         self.assertEqual((originals, missing), ({"list.txt": "two"}, []))
 
+    def test_an_empty_or_html_original_fails_the_platform(self):
+        """A System.dat served as an HTML page with 200, or a 0-byte cache,
+        was parsed as a file with no entries: four exporters rebuilt it from
+        our data and reported OK."""
+        for body in ("", "  \n", "<html><body>Too Many Requests</body></html>",
+                     "<!DOCTYPE html>\n<html></html>"):
+            with self.subTest(body=body):
+                url = self._upstream("served.txt", body)
+                originals, missing = collect_originals(
+                    _OneFileExporter(url), {}, self.cache, True, refresh=True
+                )
+                self.assertEqual(originals, {})
+                self.assertEqual(len(missing), 1)
+                cached = self.cache / "plat" / "list.txt"
+                cached.parent.mkdir(parents=True, exist_ok=True)
+                cached.write_text(body, encoding="utf-8")
+                _, missing = collect_originals(
+                    _OneFileExporter(url), {}, self.cache, False
+                )
+                self.assertEqual(len(missing), 1)
+                cached.unlink()
+
     def test_offline_serves_what_is_cached_and_names_what_is_not(self):
         url = self._upstream("v1.txt", "one")
         _, missing = collect_originals(_OneFileExporter(url), {}, self.cache, False)