Allow overwrite of constructor arguments to return None

[oruebel]

When customizing in the ObjectMapper the mapping for a constructor argument, the mapping is ignored if the function returns None. E.g.,

@register_map(Sweeps)
class SweepsMap(DynamicTableMap):
    @ObjectMapper.constructor_arg('intracellular_recordings')
    def intracellular_recordings_arg(self, builder, manager):
          return None

will always be ignored. This is due to the following:

hdmf/src/hdmf/build/map.py

Lines 1204 to 1210 in 44c02ea

	override = self.__get_override_carg(argname, builder, manager)
	if override is not None:
	val = override
	elif argname in const_args:
	val = const_args[argname]
	else:
	continue

Here the check if override is not None: is used to check if a constructor argument is overwritten or not. Instead the check should look for whether a custom mapping is defined, rather than using the None return value as an indicator that there is no overwrite.

Proposed solution Add function has_overwrite_carg to check if a constuctor argument is overwritten and use it instead.

Checklist

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[rly]

This is a quick fix. Will do and add test for next release.

[mavaylon1]

@rly is this something that we want before the major release?