Remove null=True from string fields

[maurizi]

We have null=True set for several string fields, which results in the empty state allowing both the empty string and null.

Which should remove null=True for these fields and write a data migration to change null values in those fields to an empty string.

Fields in question:

cac-tripplanner/python/cac_tripplanner/destinations/models.py

Line 117 in 1b81ce2

website_url = models.URLField(blank=True, null=True)

cac-tripplanner/python/cac_tripplanner/destinations/models.py

Lines 163 to 168 in 1b81ce2

	zipcode = models.CharField(max_length=5, null=True)
	# In the admin interface, display the address right above the map, since it triggers geocoding
	address = models.CharField(max_length=40, null=True,
	help_text=('The map automatically updates as the address is typed, '
	'but may be overridden manually if incorrect.'))