Resolvers: Create tests for createPost.js

[palisadoes]

The Talawa-API code base needs to be 100% reliable. This means we need to have 100% test code coverage.

Tests need to be written for file lib/resolvers/post_mutations/createPost.js

• We will need the API to be refactored for all methods, classes and/or functions found in this file for testing to be correctly executed.
• When complete, all all methods, classes and/or functions in the refactored file will need to be tested. These tests must be placed in a
  single file with the name tests/resolvers/post_mutations/createPost.spec.js. You may need to create the appropriate directory structure to do this.

IMPORTANT:

Please refer to the parent issue on how to implement these tests correctly:

• Create all API Resolver Tests #490

PR Acceptance Criteria

• When complete this file must show 100% coverage when merged into the code base. This will be clearly visible when you submit your PR.
• The current code coverage for the file can be found here. If the file isn't found in this directory, then tests have not been created.
• The PR will show a report for the code coverage for the file you have added. You can use that as a guide.