Don't comment "; Next fragment/union" in .map files for empty section pieces

Fixes #1821

test/link/fragment-literals/a.asm

@@ -0,0 +1,18 @@
+section "PrintHL", rom0
+PrintHL::
+	; ...
+	ret
+
+section "PrintNth", rom0
+PrintNth::
+	ld hl, [[
+		dw [[ db "one\0" ]]
+		dw [[ db "two\0" ]]
+		dw [[ db "three\0" ]]
+		dw [[ db "four\0" ]]
+	]]
+	ld d, 0
+	ld e, a
+	add hl, de
+	add hl, de
+	call PrintHL