Jet color scheme is all black (blue?) when inverted

[alanocallaghan]

Before we begin...

Before submitting your bug report, please check the following:

• I've definitely found a bug (it you're not sure, please use image.sc instead)
• I've checked https://qupath.github.io for a new release that might already have fixed the issue
• I've checked the Changelog to see if the bug has already been fixed in the next release
• I've checked for existing GitHub issues describing the same problem

Bug report

Describe the bug
When using Jet (legacy), the ColorMap returns the first RGB map value for all values if max < min.

Reported here first: https://forum.image.sc/t/qupath-lut-change-in-measurmernts-maps/92030/4

To Reproduce
Steps to reproduce the behavior:

1. Create detections
2. Open measurement maps
3. Select jet color scheme
4. Move max above min

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots

Regular:

Inverted:

Desktop:

• OS: Linux (any though)
• QuPath Version: [e.g. 0.5.0]

Additional context
PR incoming, this is just to document the behaviour