[MRG] get_backend compatibility with None entries

[eloitanguy]

Types of changes

Tweaked the code for get_backend so as to ignore None entries

Motivation and context / Related issue

This is a convenience addition for optional arguments:

def f(a, b=None):
     nx = get_backend(a, b)

will be nicer that

def f(a,b=None):
     if b is None:
          nx = get_backend(a)
     else:
          nx = get_backend(a, b)

especially in the case where there are a lot of optional arrays, where the amount of required if statements is combinatorial.

How has this been tested (if it applies)

Added a test in test/test_backend.py. Ran and passed the test workflow.

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• [] All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

Merging #525 (10f69ed) into master (064898d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #525   +/-   ##
=======================================
  Coverage   96.06%   96.06%           
=======================================
  Files          65       65           
  Lines       13725    13732    +7     
=======================================
+ Hits        13185    13192    +7     
  Misses        540      540