Do not swallow StatisticsError

[Masynchin]

This AverageIndent swallows exception when its lines empty:

ndnt/ndnt/indent.py

Lines 28 to 41 in 091c121

	class AverageIndent:
	"""Average indent of lines block."""
	def __init__(self, origin: Lines):
	self.origin = origin
	def value(self) -> float:
	"""Average indent itself."""
	indents = map(Indent, self.origin)
	try:
	return mean(indent.length() for indent in indents)
	except StatisticsError:
	# if origin has no lines
	return 0.0

This is why we can't either filter out empty indents or do zero by ourselves.

Goals:

• Move exception handling to another class