Fix all-`Categorical`/`BayesianOptPro` bug

[codecov[bot]]

Codecov Report

Merging #156 into master will increase coverage by 0.09%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #156      +/-   ##
=========================================
+ Coverage   93.61%   93.7%   +0.09%     
=========================================
  Files          46      46              
  Lines        4837    4829       -8     
=========================================
- Hits         4528    4525       -3     
+ Misses        309     304       -5

Impacted Files	Coverage Δ
hyperparameter_hunter/space/space_core.py	94.15% <ø> (ø)	:arrow_up:
...yperparameter_hunter/optimization/protocol_core.py	96.27% <ø> (+1.58%)	:arrow_up:
...er_hunter/optimization/backends/skopt/protocols.py	100% <ø> (ø)	:arrow_up:

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[coveralls]

Pull Request Test Coverage Report for Build 691

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 93.705%

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Totals
Change from base Build 687:	0.09%
Covered Lines:	4525
Relevant Lines:	4829

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💛 - Coveralls