lda #<EmptyString  ;address of string we wish to check
sta $00
lda #>EmptyString
sta $01            ;the empty string has been assigned to zero page pair $00 and $01

ldy #0
ldx #0
getStringLength:
lda ($00),y
beq Terminated
iny
jmp getStringLength
Terminated:
cpy #0
beq StringIsEmpty ;if this branch is taken, the string is empty
                  ;otherwise, the string is not empty