only create fallback content if no slot is provided

[Rich-Harris]

fixes #2354

[codecov-io]

Codecov Report

Merging #2409 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2409   +/-   ##
=======================================
  Coverage   91.83%   91.83%           
=======================================
  Files           1        1           
  Lines          49       49           
=======================================
  Hits           45       45           
  Misses          4        4

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