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-include.out

@@ -0,0 +1,19 @@
+Within REPT
+Within INCLUDE: _u1
+Outside INCLUDE: _u1
+Within REPT
+Within INCLUDE: _u2
+Outside INCLUDE: _u2
+Within MACRO
+Within INCLUDE: _u3
+Outside INCLUDE: _u3
+
+Within REPT: _u4
+Within INCLUDE: _u4
+Outside INCLUDE: _u4
+Within REPT: _u5
+Within INCLUDE: _u5
+Outside INCLUDE: _u5
+Within MACRO: _u6
+Within INCLUDE: _u6
+Outside INCLUDE: _u6