Some logic clean up and better with defense programming.

[jcs090218]

This patch does the following changes.

For imenu

• Some logic clean up
• Use defense programming strategy to lsp-ui-imenu--get-padding.
• Supply missing docstring. (By my understanding)

For sideline

• Some operations changed.
• Removed unnecessary newlines that may cause confusions.

[jcs090218]

I think this is a simple patch with code improvement. Merge this now. :)