Python: Call analysis fails in some scenarios

While trying to use the 'pointsTo' approach for some basic control-flow-based queries (please let me know if there's a better approach to find 'all statements reachable from a function entry-point'), I've noticed that currently, some calls are incorrectly picked up without any value to point to.

MWE:
```python
import sys

def mwe_callable():
print("Hello, World!") # works

def mwe_broken():
if herp := sys.argv[1]:
raise Exception("merp") # broken

mwe_callable() # broken

def mwe_broken2():
if herp := "derp":
print("merp") # broken

mwe_callable() # works

def mwe_works():
if sys.argv[1] == "derp":
raise Exception("merp") # works

mwe_callable() # works

def mwe_works2():
print("merp") # works
mwe_callable() # works
```

Test query:
```codeql
import python

from Function f, Call c, Expr e
where
f.contains(c) and
e = c.getFunc() and
not exists(Value v | e.pointsTo() = v)
select f, c, e
```

Every call I've marked here as 'broken' is returned by the query as not having any Value to point to, whereas the other calls are correctly identified and associated with their target.

I am using:
- CodeQL CLI 2.21.0
- CodeQL VSCode extension 1.17.2
- codeql/python-all@4.0.4
- Python extractor 1.22.1

まず、提供された CodeQL クエリを最小限の Python の例に対して実行し、動作するとマークされた呼び出しと壊れているとマークされた呼び出しを比較します。代入式に対する Python extractor と pointsTo の動作を確認し、ターゲットが Value を持つ呼び出しをクエリが報告しなくなったことを確認します。

索引モデルが issue の本文から書いたものです。