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

[gkalpak]

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

[gkalpak]

With this PR, the warning about no matching when expression is only logged if the new value is not null/undefined. I thought logging even for null/undefined would produce too many (possibly undesirable) warnings.
But I don't feel to strongly about it though.

/ping @lgalfaso _{(because if I recall correctly you were the one "mentoring" the relevant PRs)}

[petebacondarwin]

LGTM