Fix malformed UTF-8 character in JSON string error

The different database drivers do not retrieve the data with the same
Perl's internal representation. Therefore it is required to check this
representation before calling "decode_json" that "expects an UTF-8
(binary) string" (https://metacpan.org/pod/JSON::PP#decode_json).

lib/Zonemaster/Backend/DB.pm

@@ -260,9 +260,11 @@ sub get_test_params {
 
     my $result;
     eval {
-        # TODO: do we use "encode_utf8" as this was the case in PostgreSQL
-        #       (see commit diff)
-        $result = decode_json( $params_json );
+        if ( utf8::is_utf8( $params_json ) ) {
+            $result = decode_json( encode_utf8( $params_json ) );
+        } else {
+            $result = decode_json( $params_json );
+        }
     };
 
     die Zonemaster::Backend::Error::JsonError->new( reason => "$@", data => { test_id => $test_id } )