Fix for Mul.is_real

[ShubhamKJha]

References to other Issues or PRs

Fixes #16313

Brief description of what is fixed or changed

Consider the following code:

>>> from sympy import symbols
>>> x = symbols('x', real=False)
>>> (-x).is_real
>>>

Apparently the negation of a non-real should be non-real but I still don't know what non-real numbers do sympy support and what symbols with real=False could imply.

Are there any non-real numbers when multiplied by -1 produces real numbers?

But by understanding the code in sympy.core.Mul.is_eval_real_imag(), I could infer that since complex = None would mean either complex=True or complex=False in both of these cases the present code was giving result False with real=False. So the desired result is also False.

Other comments

Release Notes

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#### References to other Issues or PRs
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Fixes #16313 

#### Brief description of what is fixed or changed
Consider the following code:
```py
>>> from sympy import symbols
>>> x = symbols('x', real=False)
>>> (-x).is_real
>>>
```
Apparently the negation of a non-real should be non-real but I still don't know what non-real numbers do sympy support and what symbols with `real=False` could imply.

`Are there any non-real numbers when multiplied by -1 produces real numbers?`

But by understanding the code in `sympy.core.Mul.is_eval_real_imag()`, I could infer that since `complex = None` would mean either `complex=True` or `complex=False` in both of these cases the present code was giving result `False` with `real=False`. So the desired result is also `False`.

#### Other comments


#### Release Notes

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[jksuom]

Can you add a unit test?

[ShubhamKJha]

@jksuom I added tests.

[jksuom]

Thanks, looks ok.

[ShubhamKJha]

Thanks @jksuom

[asmeurer]

Please include release notes entries in the future.