Description

No change to functionality; just a small tidy. Seems a bit pointless, but it will make it easier to clean up some tests in a following PR...

• prefer to use None over html.Div(hidden=True) in the case that we don't need to refer to the element in any way (basically whenever you don't set an id). e.g. html.P(self.title) if self.title else None
• prefer to use html.Div(hidden=True) over None in the case that we do need to refer to the element (basically when you do set an id). e.g. html.Div(hidden=True, id="nav_panel_outer"). Generally these can be identified by the fact that build return values have a type like _NavBuildType
• prefer to use "" as default value for optional fields which are str. These fields do not need to accept None values at all

Checklist

• I have not referenced individuals, products or companies in any commits, directly or indirectly
• I have not added data or restricted code in any commits, directly or indirectly
• I have updated the docstring of any public function/class/model changed
• I have added tests to cover my changes (if applicable)

Types of changes

• Docs/refactoring (non-breaking change which improves codebase)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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