Magic test for archives with zero-length entry

Noticed when attempting to unpack a python `.whl` wheel file;
They're just renamed .zip's, and usually work OK, but there are casess
where the first file in the `.whl` archive is zero bytes, and that
causes `file -z` to error:

```bash
❯ file -zL tests/test-1.23.zip
tests/test-1.23.zip: ERROR:[gzip: ] (data)

❯ file tests/test-1.23.zip
tests/test-1.23.zip: Zip archive data, made by v2.3 UNIX, extract using at least v1.0, last modified Oct 28 2006 14:38:44, uncompressed size 0, method=store

❯ unzip -l tests/test-1.23.zip
Archive:  tests/test-1.23.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
        0  2006-10-28 14:38   1/2/3
        0  2006-10-28 14:38   a/b
        0  2006-10-28 14:38   foobar
---------                     -------
        0                     3 files
```

This means we were not using `file` magic for any archive that started
with a zero-length file (or directory) entry.

Add a fallback when the magic test fails in this case, and add a test to
cover it.

Author: noahp

dtrx/dtrx.py

@@ -1357,11 +1357,22 @@ def try_by_magic(self, filename):
             status = process.wait()
             if status != 0:
                 return []
+            # if output contains 'ERROR:[', there was an error unzipping the
+            # first archive entry. re-run without -z.
+            output = process.stdout.readline().decode("ascii")
+            process.stdout.close()
+            if "ERROR:[" in output:
+                process.stdout.close()
+                process = subprocess.Popen(["file", "-L", filename], stdout=subprocess.PIPE)
+                status = process.wait()
+                if status != 0:
+                    return []
+                output = process.stdout.readline().decode("ascii")
+                process.stdout.close()
+
         except FileNotFoundError:
             logger.error("'file' command not found, skipping magic test")
             return []
-        output = process.stdout.readline().decode("ascii")
-        process.stdout.close()
         if output.startswith("%s: " % filename):
             output = output[len(filename) + 2 :]
         mimes = self.magic_map_matches(output, self.magic_mime_map)

tests/test-1.23.whl

@@ -987,3 +987,10 @@
   filenames: getting-started.crx
   baseline: |
     unzip -q $1 -d getting-started
+
+- name: whl
+  filenames: test-1.23.whl
+  baseline: |
+    mkdir test-1.23
+    cd test-1.23
+    unzip -q ../$1