Add empty data directive warning

Fixes #516

test/asm/empty-data-directive.asm

@@ -0,0 +1,16 @@
+SECTION "Empty Data Directive in ROM", ROM0
+	ds 1
+	ds 2
+	ds 3
+	ds 4
+	db
+	dw
+	dl
+SECTION "Empty Data Directive in HRAM", HRAM
+	ds 1
+	ds 2
+	ds 3
+	ds 4
+	db
+	dw
+	dl

test/asm/empty-data-directive.err

@@ -0,0 +1,6 @@
+warning: empty-data-directive.asm(6): [-Wempty-data-directive]
+    db/dw/dl directive without data in ROM
+warning: empty-data-directive.asm(7): [-Wempty-data-directive]
+    db/dw/dl directive without data in ROM
+warning: empty-data-directive.asm(8): [-Wempty-data-directive]
+    db/dw/dl directive without data in ROM