eslint-plugin-unicorn: `prefer-string-starts-ends-with`: incorrect autofix when string does not exist

Problem

If the string variable is null or undefined, the autofix will cause an exception.

Before autofix:

/^en/.test(lang)
// Returns `false` if `lang` is `undefined` / `null`

After autofix:

lang.startsWith('en')
// Uncaught TypeError: Cannot read property 'startsWith' of undefined

Fix

A year from now when we drop support for Node 12, we can use optional chaining to autofix this safely:

lang?.startsWith('en')

Until then, we have a few options:

1. Do nothing since the autofixer has been around or a year (#711) and no one else has complained
2. Temporarily turn the autofixer into an automated suggestion until we can use the optional chaining autofix
3. Warn about this situation in the rule doc