Inconsistency between result.stdout and error.stdout.

[ehmicky]

We've been trying to keep the shape of an Execa's successful result and an error as close to each other as possible.

At the moment, result.stdout is:

• undefined when:
  • childProcess.stdout is undefined, e.g. when using stdout: 'ignore'
  • buffer is false
• an empty string when there is an stdout but it is empty

Distinguishing between those two is useful. However, error.stdout does not do this: instead of undefined, it is an empty string.
We should fix this and make error.stdout behave like result.stdout.

This also applies to result.stderr and result.all.

We should also fix its types as suggested in #673.

[ehmicky]

Done by #686.