Throwing an error when python-version input is empty and PYTHON_VERSION_REQUIRED is set true

[kmasuda050]

Description:

• The error message has been added in Add warning if python version set to empty value #396 . Whether the warning message is displayed or exited can be changed by PYTHON_VERSION_REQUIRED environment variable.

Related issue:

• Zero exit with empty input for python-version #241

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.