Fix a mypy warning

Author: jairov4

mypyc/irbuild/ll_builder.py

@@ -1597,8 +1597,8 @@ def unary_op(self, value: Value, expr_op: str, line: int) -> Value:
             return Float(-value.value, value.line)
         if isinstance(typ, RInstance):
             result = self.dunder_op(value, None, expr_op, line)
-            assert result, "Unsupported unary operation: %s" % expr_op
-            return result
+            if result is not None:
+                return result
         call_c_ops_candidates = unary_ops.get(expr_op, [])
         target = self.matching_call_c(call_c_ops_candidates, [value], line)
         assert target, "Unsupported unary operation: %s" % expr_op