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'