Exception when templates.json exists, but is empty

```
2020-11-02 15:57:06 [ERROR]: [Error: Could not load .aws/templates.json: Error: Could not parse .aws/templates.json: value expected at offset 0, length 0
at /Users/richali/aws-toolkit-vscode/dist/src/lambda/config/templates.js:114:19
at Generator.next ()
at fulfilled (/Users/richali/aws-toolkit-vscode/dist/src/lambda/config/templates.js:9:58)]
2020-11-02 15:57:06 [ERROR]: An error occurred while deploying a SAM Application. Check the logs for more information by running the "View AWS Toolkit Logs" command from the Command Palette.
```

Repro:
* `cat /dev/null > /.aws/templates.json`
* attempt to deploy template with parameters that do not have default values

Start with the stack trace location in dist/src/lambda/config/templates.js at line 114 and reproduce the deployment after creating an empty .aws/templates.json with cat /dev/null. Trace how the empty file is parsed during SAM deployment, then verify that the deployment no longer fails solely because templates.json is empty.

Written by the indexing model from the issue text.