fix(ngPluralize): fix wrong text content when count is null/undefined

When `lastCount` was evaluated to an non-numeric value (e.g. "other") and
`count` was evaluated to `NaN` (e.g. `null`/`undefined`), the text content
would be (wrongly) based on the previous template.
This commits makes sure the text content is updated correctly.

In order to customize the message shown upon `null`/`undefined` one can
specify a `'NaN'` property on the `when` exression object.

Closes #10836

Closes #10841