fix(auth): use basic status instead deprecated empty string

[smorcuend]

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Short description of what this resolves:

On Auth components with https://github.com/akveo/nebular/pull/2047/files# changes:
Warning message is triggered when nbInput component with status input attribute is an empty string.
On login component there is an occurrence with this issue. Replace empty string with basic value on status attribute.

[yggg]

Thanks @smorcuend! 👍