Fix non-deterministic test failure in test_gravity_term

[traversaro]

This PR fixes the non-deterministic test failure discussed in conda-forge/staged-recipes#21894 (comment) and #38 .
The source of the non-determinism was the use of non-initialized memory, as a idyntree.bindings.Twist object was created, but never initialized to zero. In most of the cases their actual value was zero, but sometime its value was actually something else, resulting in a test failure. See the following code snippet for an example:

>>> import idyntree
>>> a = idyntree.bindings.Twist()
>>> a.toString()
'4.64577e-310 0 6.91004e-310  6.91004e-310 0 2.07508e-322 \n'
>>> a = idyntree.bindings.Twist.Zero()
>>> a.toString()
'0 0 0  0 0 0 \n'

[Giulero]

Thanks a lot @traversaro for spotting and solving this issue!