fix: support no configuration file for the CLI

Update the CLI to work if no configuration file is found.

This is the simplest fix I could think of. Theoretically the problem
could be addressed in the API (svglint.js line 137 and 150) by replacing
the default parameter `config={}` with an implementation that defaults
to an empty object for both `undefined` **and** `null`. However, it's
currently not clear that's the right move.

Another alternative would be for `loadConfigurationFile` to return
`undefined` instead of `null`. However, that could interfere with the
intended behaviour of that function, making it impossible to distinguish
a missing default export from no-configuration-file-found.

bin/cli.js

@@ -83,6 +83,8 @@ process.on("exit", () => {
             if (cli.flags.config) {
                 logger.error("Configuration file not found");
                 process.exit(EXIT_CODES.configuration);
+            } else {
+                configObj = {};
             }
         }
     } catch (e) {