SQL programmers tend to trust the optimizer rather than write programs
that change on the fly. We also like the easiest to read form of
identical expressions:

county LIKE COALESCE (@my_county, '%')

Go with this and let the compiler figure out if the parameter is a
NULL, a string or a pattern. The LIKE predicate generates a simple
finite state machine to parse a string using the pattern given. The
state machine for '%' is very fast.