[minidom] add missing return types

[FrancescElies]

Adds explicit return types for 4 functions

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pytest (https://github.com/pytest-dev/pytest)
+ testing/test_junitxml.py:1491: error: "Node" has no attribute "getAttribute"  [attr-defined]
+ testing/test_junitxml.py:1492: error: "Node" has no attribute "getAttribute"  [attr-defined]
+ testing/test_junitxml.py:1520: error: "Node" has no attribute "getAttribute"  [attr-defined]

cwltool (https://github.com/common-workflow-language/cwltool)
+ cwltool/cuda.py: note: In function "cuda_version_and_device_count":
+ cwltool/cuda.py:21:13: error: Item "Node" of "Optional[Node]" has no attribute "data"  [union-attr]
+ cwltool/cuda.py:21:13: error: Item "None" of "Optional[Node]" has no attribute "data"  [union-attr]

[srittau]

The new pytest primer hits are probably because getElementsByTagName should return NodeList[Element], instead of NodeList[Node]. The new cwltool hit seems to point to a genuine problem.