Each character string constant appearing outside a DATA statement is followed by a null character to ease communication with C routines. You can make character string constants consisting of no characters, but only as arguments being passed to a subprogram. Such zero length character string constants are not FORTRAN standard.
demo% cat NulChr.f write(*,*) 'a', '', 'b' stop end demo% f77 NulChr.f NulChr.f: MAIN: demo% a.out ab demo%
However, if you put such a null character constant into a character variable, the variable will contain a blank, and have a length of at least 1 byte.
demo% cat NulVar.f character*1 x / 'a' /, y / '' /, z / 'c' / write(*,*) x, y, z write(*,*) len( y ) end demo% f77 NulVar.f NulVar.f: MAIN: demo% a.out a c 1 demo%