chg: Return empty list instead of None

[zaphodef]

In all cases but one, the 3rd returned object is a (potentially empty) list.

[coveralls]

Coverage remained the same at 38.286% when pulling a2aa564 on zaphodef:patch-1 into 783b848 on MISP:master.

[coveralls]

Coverage remained the same at 38.286% when pulling a2aa564 on zaphodef:patch-1 into 783b848 on MISP:master.

[coveralls]

Coverage remained the same at 38.286% when pulling a2aa564 on zaphodef:patch-1 into 783b848 on MISP:master.

[codecov]

Codecov Report

Merging #434 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #434   +/-   ##
=======================================
  Coverage   38.28%   38.28%           
=======================================
  Files          35       35           
  Lines        6138     6138           
=======================================
  Hits         2350     2350           
  Misses       3788     3788

Impacted Files	Coverage Δ
pymisp/tools/create_misp_object.py	21.62% <0%> (ø)	⬆️

---

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[codecov]

Codecov Report

Merging #434 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #434   +/-   ##
=======================================
  Coverage   38.28%   38.28%           
=======================================
  Files          35       35           
  Lines        6138     6138           
=======================================
  Hits         2350     2350           
  Misses       3788     3788

Impacted Files	Coverage Δ
pymisp/tools/create_misp_object.py	21.62% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 783b848...a2aa564. Read the comment docs.

[Rafiot]

Good catch!