ast: FormattedValue.conversion cannot be None (#7810)

```
In [4]: ast.dump(ast.parse('f"{x!r}"'))
Out[4]: "Module(body=[Expr(value=JoinedStr(values=[FormattedValue(value=Name(id='x', ctx=Load()), conversion=114)]))], type_ignores=[])"

In [5]: ast.dump(ast.parse('f"{x}"'))
Out[5]: "Module(body=[Expr(value=JoinedStr(values=[FormattedValue(value=Name(id='x', ctx=Load()), conversion=-1)]))], type_ignores=[])"
```
(On 3.9 but I don't think this has changed since 3.6.)

The stdlib also assumes this: https://github.com/python/cpython/blob/main/Lib/ast.py#L1211 (`chr(None)` doesn't work).

stdlib/_ast.pyi

@@ -319,7 +319,7 @@ class FormattedValue(expr):
     if sys.version_info >= (3, 10):
         __match_args__ = ("value", "conversion", "format_spec")
     value: expr
-    conversion: int | None
+    conversion: int
     format_spec: expr | None
 
 class JoinedStr(expr):