You can do this by the use of If...then...Else field constructions in the
mail merge main documnet

e.g

{ IF { MERGEFIELD Title } = "" "{ MERGEFIELD FirstName }" "{ MERGEFIELD
Title } { MERGEFIELD FirstName }" }

and

{ IF { MERGEFIELD Company } = "" "{ MERGEFIELD Address1 }" "{ MERGEFIELD
Company }[Enter]
{ MERGEFIELD Address1 }" }

You must use Ctrl+F9 to insert each paird of field delimiters { } and in
place of the [Enter] in the second construction, you press the Enter key (or
Shift+Enter) at that point so that the balance of the field construction is
on a new line.

Use Alt+F9 to toggle off the display of the field codes.