Prevent passing assertions that RGBASM passed to RGBLINK

test/asm/assert-const.asm

@@ -0,0 +1,7 @@
+; The following link-time assertion is known by RGBASM to be okay.
+; This previously caused it to still pass the assertion to RGBLINK with an empty
+; RPN expression
+
+SECTION "rgbasm passing asserts", ROM0[0]
+	db 0
+	assert @