Only restore parent context's \@ value if it had one defined (#1366)

This way, if a child context initializes `\@`, the parent won't
reset it. And if the child context did not initialize `\@`,
then resetting it would be redundant.

test/asm/unique-id-nested.asm

@@ -0,0 +1,33 @@
+MACRO m1
+	PRINTLN "Begin MACRO"
+	DEF nested EQUS """MACRO mm
+		PRINTLN "Within nested MACRO: \\@"
+		\n ENDM
+		mm"""
+	nested
+	PURGE nested, mm
+	PRINTLN "Within MACRO: \@"
+ENDM
+REPT 2
+	PRINTLN "Begin REPT"
+	m1
+	PRINTLN "Within REPT: \@"
+ENDR
+
+PRINTLN
+
+MACRO m2
+	PRINTLN "Begin MACRO: \@"
+	DEF nested EQUS """MACRO mm
+		PRINTLN "Within nested MACRO: \\@"
+		\n ENDM
+		mm"""
+	nested
+	PURGE nested, mm
+	PRINTLN "Within MACRO: \@"
+ENDM
+REPT 2
+	PRINTLN "Begin REPT: \@"
+	m2
+	PRINTLN "Within REPT: \@"
+ENDR