If landlord-mailing-address has no landlord name, the page will crash

[toolness]

Introduced in #1253.

Similarly to #1227, if the NoRent page in which we ask the user for their LL's mailing address is used when the user hasn't entered the LL's name for some reason, the page will explode because we're putting the name in a hidden field, and it's expected to be non-blank.