MAINT: f2py: don't generate code that triggers `-Wsometimes-uninitialized` (#20940)

* MAINT: f2py: don't  generate code that triggers `-Wsometimes-uninitialized`

Warnings look like:

```
scipy/linalg/_flapackmodule.c:2200:9: warning: variable 'return_value' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
    if (capi_j>capi_i)
        ^~~~~~~~~~~~~
scipy/linalg/_flapackmodule.c:2217:12: note: uninitialized use occurs here
    return return_value;
           ^~~~~~~~~~~~
scipy/linalg/_flapackmodule.c:2200:5: note: remove the 'if' if its condition is always true
    if (capi_j>capi_i)
    ^~~~~~~~~~~~~~~~~~
scipy/linalg/_flapackmodule.c:2099:21: note: initialize the variable 'return_value' to silence this warning
    int return_value;
                    ^
                     = 0
```

Also:

- Initialize complex return value.
- Warn on non-void callback returning None.
- Use brackets in if-else block.

This makes the code more readable.

Co-authored-by: Pearu Peterson <pearu.peterson@gmail.com>

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