test/assemblers: omit None compression option

In the "test_tar" test the compression can be None, which in turn
will be serialized as "compression: null" in the options dict.
However, this is not a valid option according to the schema. The
schema could be adapted to allow for this but it is probably
better to just omit empty or null values.

test/test_assemblers.py

@@ -107,10 +107,9 @@ class TestAssemblers(osbuildtest.TestCase):
             ("tree.tar.gz", "gzip", ["application/x-gzip", "application/gzip"])
         ]
         for filename, compression, expected_mimetypes in cases:
-            options = {
-                "filename": filename,
-                "compression": compression
-            }
+            options = {"filename": filename}
+            if compression:
+                options["compression"] = compression
             _, output_id = self.run_assembler("org.osbuild.tar", options)
             image = f"{self.store}/refs/{output_id}/{filename}"
             output = subprocess.check_output(["file", "--mime-type", image], encoding="utf-8")