Move GlobalBackend check to backends.__init__.py

[renatomello]

This replaces all the repeated lines like:

if backend is None:  # pragma: no cover
    backend = GlobalBackend()

by a function in backends.__init__.py

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f67dc84) 100.00% compared to head (6321293) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1193   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           69        69           
  Lines        10107     10166   +59     
=========================================
+ Hits         10107     10166   +59     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[alecandido]

The change itself is good to me, this pattern was really ubiquitous.

[MatteoRobbiati]

Thanks.