fix: settle export sizes, never write None

A truth that accepts several sizes left an export with no size even
where the platform states one of them for the same dump, and RomM's
fixture then received the string None, which RomM cannot parse. A
size list now admits the platform's own size, RomM keeps its value
when nothing settles one, and validation requires a number.

scripts/exporter/romm_exporter.py

@@ -127,7 +127,13 @@ class Exporter(BaseExporter):
                     continue
                 entry: OrderedDict[str, str] = OrderedDict()
                 # The fixture states every value as a string, size included.
-                entry["size"] = str(fe.size())
+                # Where the model settles no size, RomM's own value stands;
+                # str(None) wrote "None", and RomM's int() on it raised.
+                size = fe.size()
+                if size is None and fe.platform is not None:
+                    size = fe.platform.get("size")
+                if size is not None:
+                    entry["size"] = str(int(size))
                 crc = fe.record_hash("crc32")
                 if crc:
                     entry["crc"] = crc
@@ -159,7 +165,7 @@ class Exporter(BaseExporter):
                 issues.append(f"key without a platform slug: {key}")
             elif not self._known_platform(slug):
                 issues.append(f"platform slug RomM does not know: {slug}")
-            if not entry.get("size"):
+            if not str(entry.get("size", "")).isdigit():
                 issues.append(f"entry without a size, never verifiable: {key}")
             if not any(entry.get(h) for h in ("md5", "sha1", "crc")):
                 issues.append(f"entry without a hash, never verifiable: {key}")