Handle case where no all-contributors table exists in file, but `.all-contributorsrc` exists in the repository

**Is your feature request related to a problem? Please describe.**
The bot currently decides whether to 'init' (install the placeholders for the table and badge) for the README if there is no `.all-contributorsrc` file. If the file is found, it skips this step. This causes the bot to crash if a user has an `.all-contributorsrc` file but has nothing else.

**Describe the solution you'd like**
Handle the case where the `.all-contributorsrc` file exist, but the contentFiles (READMEs) have not been setup yet.

**Describe alternatives you've considered**
Better documentation?

**Additional context**
Code that determines whether to 'init'/setup/add place holders to contentFiles (READMEs):
https://github.com/all-contributors/all-contributors-bot/blob/master/src/tasks/processIssueComment/probot-processIssueComment.js#L50-L53

```
await contentFiles.fetch(optionsConfig)
if (optionsConfig.getOriginalSha() === undefined) {
contentFiles.init()
}
```

Table placeholders that are inserted into files during 'init':
```

```