Blank variables with filters use first filter as variable name

For example, ` | append 'foo'` will try to look up the variable name "append" rather than append "foo" to "".
```diff
--- expected
+++ actual
@@ -1 +1 @@
-nil
+#<Liquid::VariableLookup:0xXXXXXX @name="flt", @lookups=[], @command_flags=0>
```