fix: strip invalid values from versions file

tests/test_legacy_version_file.py

@@ -0,0 +1,25 @@
+from typing import TYPE_CHECKING
+
+from dzgui.init.migrate import clean_legacy_versions_file
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+def test_null_signature_stripping(tmp_path: "Path") -> None:
+    tmp = tmp_path.joinpath("dzg.versions")
+    lines = [
+        "1111,2222",
+        "3333,4444",
+        "5555,null",
+        "foo",
+        ",4555",
+        "1111",
+        " ",
+        ",,,",
+        "-1",
+    ]
+    expect = ["1111,2222", "3333,4444"]
+    tmp.write_text("\n".join(lines))
+    clean_legacy_versions_file(tmp)
+    assert expect == tmp.read_text().splitlines()