Remove casting of user and item IDs in Spark evaluation

[anargyri]

Description

User and item IDs should not be cast into double (if they are strings then this turns them into null values).

Related Issues

Checklist:

• I have followed the contribution guidelines and code style for this project.
• I have added tests covering my contributions.
• I have updated the documentation accordingly.
• This PR is being made to staging branch and not to main branch.

[codecov-commenter]

Codecov Report

Merging #1686 (f41dbd4) into staging (6eb39ff) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           staging    #1686      +/-   ##
===========================================
+ Coverage    23.08%   23.10%   +0.02%     
===========================================
  Files           88       87       -1     
  Lines         9120     9110      -10     
===========================================
  Hits          2105     2105              
  Misses        7005     7005              

Flag	Coverage Δ
nightly	?
pr-gate	23.10% <ø> (ø)

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Impacted Files	Coverage Δ
recommenders/evaluation/spark_evaluation.py	86.60% <ø> (ø)
recommenders/__init__.py

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[miguelgfierro]

LGTM