Fix a check for empty input in macroexpand

The equality check hasn't done the right thing since HyExpression became tuple-backed (#1804).

NEWS.rst

@@ -13,6 +13,8 @@ Bug Fixes
 * Improved support for nesting anaphoric macros by only applying
   symbol replacement where absolutely necessary.
 * Quoted f-strings are no longer evaluated prematurely.
+* Fixed a regression in the production of error messages for empty
+  expressions.
 
 0.18.0
 ==============================

hy/macros.py

@@ -308,10 +308,7 @@ def macroexpand(tree, module, compiler=None, once=False):
 
     assert not compiler or compiler.module == module
 
-    while True:
-
-        if not isinstance(tree, HyExpression) or tree == []:
-            break
+    while isinstance(tree, HyExpression) and tree:
 
         fn = tree[0]
         if fn in ("quote", "quasiquote") or not isinstance(fn, HySymbol):