Only define @ and _NARG when they have values (#1073)

Fixes #1069

Co-authored-by: Eldred Habert <eldredhabert0@gmail.com>

test/asm/undefined-builtins.asm

@@ -0,0 +1,21 @@
+; not inside a section
+assert !DEF(@)
+println @
+println "{@}?"
+
+; not inside a macro
+assert !DEF(_NARG)
+println _NARG
+println "{_NARG}?"
+
+SECTION "s", ROM0[$42]
+assert DEF(@)
+println @
+println "{@}!"
+
+MACRO m
+	assert DEF(_NARG)
+	println _NARG
+	println "{_NARG}!"
+ENDM
+	m 1, 2, 3