Fix data type issue in _compute_doppler_shifts, fixes #426

[Jeija]

Description

Very minor one-line fix for issue #426.
The line in the source code used to say

tf.where(objects_mask, 0, doppler)

which causes a TypeError (TypeError: Input 'e' of 'SelectV2' Op has type float32 that does not match type int32 of argument 't'.), because 0 is an int32, whereas doppler is a float32 and tf.where wants to have same dtypes (at least when using @tf.function with my TensorFlow installation).
I changed it so that the dtype of 0 is chosen based on the dtype of doppler:

doppler = tf.where(objects_mask, tf.constant(0, doppler.dtype), doppler)

Not sure if this is the correct way to fix it, but the bug certainly disappears for me.

Checklist

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[SebastianCa]

Hi @Jeija,
thank you for this pull request. We will merge it into the next release of Sionna.