Those functions were added in #8277 but they did more harm than good. They're subtle, don't do what people think and requires users to understand the non-uniqueness of polar decomposition. They ended up confusing people.
Until we store additional information enough to make a unique polar decomposition, these functions shouldn't be a part of Basis.
Currently we rely on some undefined behavior when Object->cast_to() gets
called with a Null pointer. This used to work fine with GCC < 6 but
newer versions of GCC remove all codepaths in which the this pointer is
Null. However, the non-static cast_to() was supposed to be null safe.
This patch makes cast_to() Null safe and removes the now redundant Null
checks where they existed.
It is explained in this article: https://www.viva64.com/en/b/0226/
