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.

.eslintrc.yml

@@ -6,8 +6,11 @@ extends:
   - plugin:@typescript-eslint/eslint-recommended
   - plugin:@typescript-eslint/recommended
   - prettier/@typescript-eslint
+parserOptions:
+  project: ['tsconfig.eslint.json']
 rules:
   # '@typescript-eslint/explicit-function-return-type': 0
   '@typescript-eslint/no-use-before-define':
     - 2
     - functions: false
+  '@typescript-eslint/no-unnecessary-condition': error