Add a default for DagModel.default_view

[kaxil]

fixes #10283

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[mik-laj]

Suggested change

	def get_default_view(self):
	"""This is only there for backward compatible jinja2 templates"""
	if self.default_view is None:
	return conf.get('webserver', 'dag_default_view').lower()
	else:
	return self.default_view
	@property
	def default_view(self):
	"""This is only there for backward compatible jinja2 templates"""
	if self._default_view is None:
	return conf.get('webserver', 'dag_default_view').lower()
	else:
	return self._default_view
	@default_view.setter
	def default_view(self, value):
	self._default_view= value

This way we won't have two similar public attributes of default_view property, get_default_view method.

[kaxil]

I kept it there for parity between Dag and DagModel, otherwise dag.html and dags.html might use or the other depending on if we are using Dag or DagModel

The other option is to create a Database migration to fill all the null values, WDYT? @mik-laj

[kaxil]

@mik-laj Any comments?

[mik-laj]

Okey. Now I understand the whole change. It looks good.