Overview
This PR fixes #11282. As per the issue, if an option is created with default value as None and a value for the option is not set in the ini file, then trying to get the option value in code with config.getini() returns an empty list [] instead of None as expected.

Analysis
On investigating it was found that the logic to return a default value for a configuration option is as follows:
if a default value is set during the creation of the option, then the set default value is returned
if a default value is NOT set during the option creation but the type for the option is provided, then return an empty list
if neither a default value nor type is provided while creating the option, then assume its a string type and return an empty string

The above logic did not seem right. Also, the logic above did not honor None as an explicit default value to return.

Solution
The solution provided in this PR will change the behavior in the following way:
if a default value is NOT set during the option creation but the type for the option is provided, then a type specific default will be returned. That is as follows:
"paths", "pathlist", "args", "linelist" types will have a default empty list [] as these are all lists
"bool" will have False as default
"string" will have empty string "" as default
logically there can't be any other type as the code checks for valid types before hand. However, just as a fail safe for future, any other types will have a default of None

if a default value is set explicitly as None during the option creation, then None will be returned as the default value irrespective of the type
if neither a default value nor type is provided while creating the option, assume its a string type and return an empty string. (This assumption stays as it is from the existing code)

Checklist

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits. Please uncheck this if you prefer to squash the commits yourself.