Fix issues in FppTest

[bocchino]

This PR fixes some issues in FppTest.

1. I noticed that the sanitizer was complaining about an array size of zero. I fixed it.
2. I noticed that a test was assuming a non-empty test string, and it failed in the case where the random test string was empty. I added the constraint that the string must have min length 1.
3. I cleaned up the logic and added some assertions in the function for generating random strings.
4. I removed trailing spaces from the files that I touched.

[LeStarch]

One minor question.