Do not raise misleading warning (#389)

When the input `input_files` is not defined, the 
`config.input_files` will default to an empty array, which always
evaluate to true - that was triggering a warning like "hey man,
the value you inputted on `input_files` didn't yield any files",
which is logically wrong if `input_files` isn't explicitly defined.

src/main.ts

@@ -60,7 +60,7 @@ async function run() {
     });
     //);
     const rel = await release(config, new GitHubReleaser(gh));
-    if (config.input_files) {
+    if (config.input_files && config.input_files.length > 0) {
       const files = paths(config.input_files);
       if (files.length == 0) {
         console.warn(`🤔 ${config.input_files} not include valid file.`);