FDA window padding effect

[jclarkeSTFC]

Found during manual testing for the 6.10 release, also present in 6.9.1.

In the FDA manual testing instructions, in the FFT Test section, it says that when the padding is zero the plot "will have lots of straight lines", but it looks like this:

Not sure if it's a real problem or maybe the instructions are just written with a little too much hyperbole.

Platform/Version (please complete the following information):

• OS: Windows
• OS Version: 10
• Mantid Version 6.9.1 and 6.9.20240514.1920