Clarify None handling for configuration parsing

[Kircheneer]

BaseSpaceConfigParser.build_config_relationship is using multiple ._build_$something methods that currently return an Optional[str]. It is not clear what happens, if one of those actually returns None. We should decide on whether we want to raise an exception in those specific methods or pass the None up the chain and handle it there. In any case, it needs to be handled to get rid of all the # type: ignore comments.

Originally posted by @Kircheneer in #123 (comment)

[Kircheneer]

Also applicable to NokiaConfigParser._get_section_title

[itdependsnetworks]

@jmcgill298 any chance you can take a look?

[jmcgill298]

The references to _build_banner all store the value as a variable, and the docstrings document expected Returns:

netutils/netutils/config/parser.py

Lines 187 to 189 in e757bfe

	Returns:
	str: The next configuration line in the configuration text.
	None: When banner end is the end of the config text.

(and replicated in other implementations)

The references to _build_nested_config all store the value as a variable, and the docstrings document expected Returns:

netutils/netutils/config/parser.py

Lines 222 to 224 in e757bfe

	Returns:
	str: The next top-level configuration line in the configuration text.
	None: When the last line of configuration text is a nested configuration line.

The references to _build_multiline_config do not store it as a variable, so that method should remove returns and always return None.

The reference to _build_multiline_single_configuration_line does not store it as a variable, so that method should remove returns and always return None.

I suspect that the last two methods had a return to simplify writing test cases