If there's not an ampersand, I want cell to be empty - it gives me #VALUE! instead

Test the & first

=IF(ISERR(FIND("&",A1,1)),"",LEFT(A1,FIND("&",A1,1)-1))

or given use of XL2007

=IFERROR(LEFT(A1,FIND("&",A1,1)-1),"")