ZDoom

I have function that collect one sided map lines and then sort them into a groups of closed outline aka group of lines that 'draw' closed polygon.
It works, but it also create groups of 'empty' sectors.
On the example of a picture it filter lines in three groups, rectangle, square and weird shape respectively.

Problem here, I dont need that square and shape for future processing, only a rectangle. How find out that group of lines is an 'empty' space and delete them?

Picture is only an example, 'real life' maps have more complex geometry.