ci-license-scan: fix failure for non-existing debian/copyright

In case if file `debian/copyright` is not existing, the `ci-license-scan`
script throws an exception. Do not stop the check if the file is absent.
Example package without the `debian/copyright` file is
`firmware-nonfree`.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

package-source-builder/overlay/usr/bin/ci-license-scan

 #!/usr/bin/env python3
 # SPDX-License-Identifier: MPL-2.0
 #
-# Copyright © 2019 Collabora Ltd
+# Copyright © 2019-2021 Collabora Ltd
 #
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -375,12 +375,15 @@ def main():
     unknown_licensed = False
     fixups = dict()
     fixups_applied = set()
-    with open('debian/copyright', 'rb') as f:
-        try:
+    try:
+        with open('debian/copyright', 'rb') as f:
             debian_copyrights = Copyright(f, strict=False)
-        except (ValueError, NotMachineReadableError):
-            print("WARN: No machine-readable debian/copyright found, can’t use licensing information from Debian.", file=sys.stderr)
-            debian_copyrights = Copyright()
+    except (ValueError, NotMachineReadableError):
+        print("WARN: No machine-readable debian/copyright found, can’t use licensing information from Debian.", file=sys.stderr)
+        debian_copyrights = Copyright()
+    except FileNotFoundError:
+        print("WARN: No debian/copyright found, can’t use licensing information from Debian.", file=sys.stderr)
+        debian_copyrights = Copyright()
     with open('debian/apertis/copyright.new', 'rb') as f:
       try:
         n = NullFilter(f)