math/big: handle additional edge cases for Rat

[mauri870]

New special cases have been added to the Rat testcase.
Updated the function quotToFloat32/64 to check for zero
as well when determining if the value is infinite. This
change ensures that the exact flag is correctly set for
these edge cases, preventing test failures.

Thanks Johan Jansons (@jupj) for the proposed fix on
the ticket description.

Fixes: #56164

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