ld hl,MyString

GetStringLength:
	ld b,0        ;zero the length counter
loop_getStringLength:
	ld a,(hl)
	or a          ;compare A to zero
	ret z         ;exit if zero

	inc hl        ;next char
	inc b         ;add 1 to length counter
	jr loop_getStringLength

ld a,b    ;load B into A
or a      ;compare A to zero (effectively comparing B to zero)
jr z,StringIsEmpty
;your code for what happens when MyString is not empty goes here.