Do not expand empty strings

Fixes #813

test/asm/expand-empty-string.asm

@@ -0,0 +1,6 @@
+test: MACRO
+v equs "X"
+X equs "" ; should not be expanded
+\1
+ENDM
+	test v 0

test/asm/expand-empty-string.err

@@ -0,0 +1,3 @@
+ERROR: expand-empty-string.asm(6) -> expand-empty-string.asm::test(4):
+    syntax error, unexpected number
+error: Assembly aborted (1 error)!

test/asm/expand-empty-string.simple.err

@@ -0,0 +1,3 @@
+ERROR: expand-empty-string.asm(6) -> expand-empty-string.asm::test(4):
+    syntax error
+error: Assembly aborted (1 error)!