[mypy] Fix type annotations for middle_element_of_linked_list.py

[archaengel]

$ git checkout master
$ mypy --ignore-missing-imports data_structures/linked_list/ > before.txt
$ git checkout mypy-fix-ll-middle-element
$ mypy --ignore-missing-imports data_structures/linked_list/ > after.txt
$ diff before.txt after.txt
1,3d0
< data_structures/linked_list/middle_element_of_linked_list.py:2: error: The return type of "__init__" must be None
< data_structures/linked_list/middle_element_of_linked_list.py:2: error: Missing return statement
< data_structures/linked_list/middle_element_of_linked_list.py:17: error: Missing return statement
22c19
< Found 21 errors in 5 files (checked 14 source files)
---
> Found 18 errors in 4 files (checked 14 source files)

 $ python3 -m doctest -v data_structures/linked_list/middle_element_of_linked_list.py
...
14 tests in 7 items.
14 passed and 0 failed.
Test passed.

Related issue: #4052

Describe your change:

Fix type annotations for middle_element_of_linked_list.py. LinkedList.middleElement's signature was changed to return Optional[int] to better reflect the absence of an element when called on an empty list. Doctests were updated to reflect change.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[archaengel]

#4927 Fixes errors in this directory.