fix save_hyperparameters(container) if container is empty

[awaelchli]

What does this PR do?

Fixes #7267

Whether this fix is correct remains to be seen ...

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[codecov]

Codecov Report

Merging #7268 (bb69009) into master (b670647) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #7268   +/-   ##
======================================
  Coverage      91%     91%           
======================================
  Files         199     199           
  Lines       12798   12800    +2     
======================================
+ Hits        11679   11681    +2     
  Misses       1119    1119           

[tchaton]

LGTM !