fix: Make MaterialSlab construction from JSON explicit

[paulgessinger]

This somehow worked for @Corentin-Allaire, but I got an error. Making it explicit should be fine.

[Corentin-Allaire]

Yes, I don't understand why the worked. I approve

[asalzburger]

If it fixes, it - let's put it in.

[codecov]

Codecov Report

Merging #704 (ef99221) into master (478a565) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #704   +/-   ##
=======================================
  Coverage   49.03%   49.03%           
=======================================
  Files         331      331           
  Lines       16549    16549           
  Branches     7725     7725           
=======================================
  Hits         8115     8115           
  Misses       3013     3013           
  Partials     5421     5421           

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