problems with list warnings

In some cases, lists shouldn't raise warnings:

```python
a1: Annotated(list[float], "m")   # should raise a warning
a2: Annotated(list[float], "m") = []  # should raise a warning
a3 = []  # should not raise a warning

x: "m"
a4 = [x]  # should raise a warning

y: "ft"
y, *_ = a4  # does not convert units

```
