Make failed macro arg expansions non-fatal

Expanding empty strings is valid but pointless;
macro args already skipped doing so, now other
`beginExpansion` calls do too.

This also fixes failed interpolations (which were
already non-fatal) to continue reading characters,
not evaluate to their initial '{' character.

test/asm/symbol-invalid-macro-arg.asm

@@ -1 +1,2 @@
-x\0
+x\0 = 10
+println x