fix: avoid displaying {{street}} when providing an empty street name

[moshikod]

when an empty (none) street value is provided, the hatool variable replacement uses the name of the field, street in this case.

the fix relies on a new calculated variable _location which is the trimmed result of concatenating the optional street name and the city.

[moshikod]

A fix for https://github.com/hasadna/avid-covider/issues/184

[akariv]

Thanks @moshikod , merged!