Is there a conditional string-concatenation ?

Hi,
Is there a conditional string-concatenation ?

I'd like to have an elegant way to connect 2 strings with some 3rd
element between only if there really are 2 strings to connect.

e.g.
MyCat ( 'John', '_', 'Doe' ) --> 'John_Doe'
while
MyCat ( 'John', '_', '' ) --> 'John'
MyCat ( '', '_', 'Doe' ) --> 'Doe'
MyCat ( '', '_', '' ) --> NULL

It should treat NULL and '' equally as empty
and it should trim each of the 3 elements.

so
MyCat ( ' John ', '_', NULL ) --> 'John'
MyCat ( 'John', NULL, 'Doe' ) --> 'JohnDoe'