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

Fixes #1069

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

test/asm/if@-no-sect.asm

@@ -1,2 +0,0 @@
-if "{@}"
-endc

test/asm/if@-no-sect.err

@@ -1,5 +0,0 @@
-error: if@-no-sect.asm(1):
-    PC has no value outside a section
-warning: if@-no-sect.asm(1): [-Wnumeric-string]
-    Treating 2-character string as a number
-error: Assembly aborted (1 error)!

test/asm/narg-nosect.asm

@@ -1 +0,0 @@
-	PRINTLN "{_NARG}"

test/asm/narg-nosect.err

@@ -1,3 +0,0 @@
-error: narg-nosect.asm(1):
-    _NARG does not make sense outside of a macro
-error: Assembly aborted (1 error)!

test/asm/narg-nosect.out

@@ -1 +0,0 @@
-$0

test/asm/pc-def.asm

@@ -1,5 +0,0 @@
-IF DEF(@)
-	PRINTLN "defined"
-ELSE
-	PRINTLN "not defined"
-ENDC

test/asm/pc-def.out

@@ -1 +0,0 @@
-defined

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

test/asm/undefined-builtins.err

@@ -0,0 +1,9 @@
+error: undefined-builtins.asm(3):
+    PC has no value outside a section
+error: undefined-builtins.asm(4):
+    Interpolated symbol "@" does not exist
+error: undefined-builtins.asm(8):
+    _NARG does not make sense outside of a macro
+error: undefined-builtins.asm(9):
+    Interpolated symbol "_NARG" does not exist
+error: Assembly aborted (4 errors)!

test/asm/undefined-builtins.out

@@ -0,0 +1,8 @@
+$0
+?
+$0
+?
+$42
+$42!
+$3
+$3!