fix: keep RetroBat's blank line between systems

RetroBat separates every system after the first with a blank line.
json.dumps dropped all 100 of them: 100 hunks of noise in a diff meant
to show corrections. The separator is written again where the original
has it, and an added system takes it too where the original separates
them all. An empty truth gives back the cached file byte for byte.

tests/test_exporters.py

@@ -1420,6 +1420,31 @@ class BizhawkExport(unittest.TestCase):
         self.assertFalse(exporter.states(entry, "sha1"))
 
 
+class RetrobatListComesBackAsWritten(unittest.TestCase):
+    """RetroBat separates each system with a blank line. json.dumps dropped
+    all 100 of them, 100 hunks of noise in a diff meant to show
+    corrections."""
+
+    def test_an_empty_truth_hands_back_the_list(self):
+        original_path = UPSTREAM_CACHE / "retrobat" / "batocera-systems.json"
+        if not original_path.exists():
+            self.skipTest("upstream cache not populated")
+        original = original_path.read_text(encoding="utf-8")
+        exporter = discover_exporters()["retrobat"]()
+        config = native_layer(load_platform_config("retrobat", str(REPO_ROOT / "platforms")))
+        systems, report = build_native_model({"systems": {}}, config)
+        produced = exporter.render(systems, report, {"batocera-systems.json": original}, config)
+        self.assertEqual(produced["batocera-systems.json"], original)
+
+    def test_an_added_system_takes_the_same_separator(self):
+        from exporter.retrobat_exporter import keep_blank_lines  # noqa: PLC0415
+
+        original = '{\n  "a": {},\n\n  "b": {}\n}\n'
+        produced = '{\n  "a": {},\n  "b": {},\n  "c": {}\n}\n'
+        self.assertEqual(keep_blank_lines(original, produced),
+                         '{\n  "a": {},\n\n  "b": {},\n\n  "c": {}\n}\n')
+
+
 class MisterDatabaseComesBackAsWritten(unittest.TestCase):
     """The BiosDB is one line of JSON. Written with indent=2 it came out as
     1499 changed lines for a database whose content had not changed."""