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/invalid-macro-arg-character.err

@@ -1,2 +1,5 @@
-FATAL: invalid-macro-arg-character.asm(1):
+ERROR: invalid-macro-arg-character.asm(1):
     Invalid character in bracketed macro argument '!'
+ERROR: invalid-macro-arg-character.asm(1):
+    syntax error, unexpected >
+error: Assembly aborted (2 errors)!