empty UV_PYTHON is used as if it's set

[bluss]

I noticed the following:

UV_PYTHON= uv init x
# error: No interpreter found for executable name `` in virtual environments, managed installations, or system path

uv 0.4.17

it goes back to previous uv versions including 0.2.x. The user expectation would probably be to not use the variable because it's empty, but I'm not sure how this is handled with other environment variables in uv.