Fix null handling, covered by integration tests

`core.getInput()` always returns a string, so testing for 'not null'
is always true. This then leads to previews set to an array with
a single empty string, breaking accept-header output.

Updated eslint rules should help avoid this issue in future, and new
integration tests verify that the github client configuration now
reflects the intended configuration options.

tsconfig.eslint.json

@@ -0,0 +1,4 @@
+{
+  "extends": "./tsconfig.json",
+  "exclude": []
+}