Avoid calling update with empty dicts

[kinow]

Was passing some functions with a debugger reading the code and noticed that here when the dict was not None, it could still be empty. And the update method was being called unnecessarily.

Won't cause any speed up, but still worth changing it I think?

[codecov]

Codecov Report

Merging #276 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #276   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          52       52           
  Lines        2624     2624           
=======================================
  Hits         2606     2606           
  Misses         18       18

Impacted Files	Coverage Δ
annif/backend/backend.py	96.42% <100%> (ø)	⬆️

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[osma]

Okay, why not. Thanks!