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

test/link/fragment-literals/ref.out.map

@@ -0,0 +1,10 @@
+SUMMARY:
+	ROM0: 39 bytes used / 16345 free
+
+ROM0 bank #0:
+	SECTION: $0000-$0025 ($0026 bytes) ["PrintNth"]
+	         $0000 = PrintNth
+	SECTION: $0026-$0026 ($0001 byte) ["PrintHL"]
+	         $0026 = PrintHL
+	EMPTY: $0027-$3fff ($3fd9 bytes)
+	TOTAL EMPTY: $3fd9 bytes

test/link/fragment-literals/ref.out.sym

@@ -0,0 +1,3 @@
+; File generated by rgblink
+00:0000 PrintNth
+00:0026 PrintHL