[lcl] Avoid crash when trying get_operations() on the LoginPage

Sometimes the LCL website does not work properly and we randomly land on
the LoginPage while trying to get transactions:
AttributeError: 'LoginPage' object has no attribute 'get_operations'

I added an "if" to verify if we are on the login page and raise
BrowserUnavailable before calling "get_operations()" to prevent these
issues that are not always reproducible.

Closes: 16243@Sibi