fix(multi-gzip) is_valid_gzip dies on empty file

Reported by @AndrewFasano:

extracting the file http://ixp.mikrotik.co.id/download/archive/6.18/all_packages-mipsbe-6.18.zip results in

2024-02-25 03:31.49 [warning  ] Unhandled Exception during multi file calculation handler=multi-gzip path=all_packages-mipsbe-6.18.zip_extract/system-6.18-mipsbe.npk_extract/4096-5779456.squashfs_v4_le_extract/home/web/webfig/index.html.gz.nocache pid=2582003 severity=<Severity.ERROR: 'ERROR'>
Traceback (most recent call last):
  File "/unblob/unblob/processing.py", line 377, in _calculate_multifile
    return dir_handler.calculate_multifile(path)
  File "/unblob/unblob/handlers/compression/gzip.py", line 179, in calculate_multifile
    if self.is_valid_gzip(file):
  File "/unblob/unblob/handlers/compression/gzip.py", line 159, in is_valid_gzip
    with File.from_path(path) as f:
  File "/unblob/unblob/file_utils.py", line 58, in from_path
    m = cls(base_file.fileno(), 0, access=access)
ValueError: cannot mmap an empty file

Author: e3krisztian

tests/handlers/compression/test_gzip.py

@@ -0,0 +1,9 @@
+from pathlib import Path
+
+from unblob.handlers.compression.gzip import MultiVolumeGzipHandler
+
+
+def test_multivolume_is_valid_gzip_empty_file(tmp_path: Path):
+    empty = tmp_path / "empty"
+    empty.touch()
+    assert not MultiVolumeGzipHandler().is_valid_gzip(empty)

unblob/handlers/compression/gzip.py

@@ -156,7 +156,12 @@ class MultiVolumeGzipHandler(DirectoryHandler):
     PATTERN = Glob("*.gz.*")
 
     def is_valid_gzip(self, path: Path) -> bool:
-        with File.from_path(path) as f:
+        try:
+            file = File.from_path(path)
+        except ValueError:
+            return False
+
+        with file as f:
             try:
                 fp = SingleMemberGzipReader(f)
                 if not fp.read_header():