fix fss node not returning T/F and not handling cases with only one f…

[perib]

What does this PR do?

FSSNode was not returning True or False for the mutation/crossover operations as required.
Additionally, when only a single feature set was passed in, there would be an error thrown. This was found in issue #1366

Where should the reviewer start?

How should this PR be tested?

subset_1_dict  = { "sense_strands" :  ['a', 'b', 'c']}
fss_1 = FSSNode(subsets=subset_1_dict)

ind1 = fss_1.generate()

for i in range(1000):
    ind1.mutate()

Before this would throw an error like :

File "/scratch2/users/mpb703/conda_environments/tpot_v1/lib/python3.10/site-packages/tpot/search_spaces/nodes/fss_node.py", line 113, in mutate self.selected_subset_name = rng.choice(names) File "numpy/random/_generator.pyx", line 803, in numpy.random._generator.Generator.choice ValueError: a cannot be empty unless no samples are take