fix lawrence_extension and lawrence_polytope for backend field

[LaisRast]

The methods lawrence extension and lawrence_polytope of Polyhedron do not work when the backend is field.

P = Polyhedron([[1,3], [3,4]], backend="field")
P.lawrence_extension([10,20])
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
TypeError: 'NoneType' object is not iterable

We fix this by properly initializing them with zero rays/lines instead of None.

CC: @jplab @kliem

Component: geometry

Keywords: polytopes, lawerence

Author: Laith Rastanawi, Jonathan Kliem

Branch/Commit: 50f9232

Reviewer: Laith Rastanawi, Jonathan Kliem

Issue created by migration from https://trac.sagemath.org/ticket/28725

[LaisRast]

Description changed:

--- 
+++ 
@@ -1,4 +1,4 @@
-The methods `lawrence extension` and `lawrence_polytope` `of Polyhedron` do not work when the backend is `field`.
+The methods `lawrence extension` and `lawrence_polytope` of `Polyhedron` do not work when the backend is `field`.
 
 ```
 P = Polyhedron([[1,3], [3,4]], backend="field")

[kliem]

New commits:

50f9232

initialize Lawrence polytope with empty list of rays/lines instead of None

[kliem]

Reviewer: Laith Rastanawi, Jonathan Kliem

[kliem]

Branch: public/28725

[kliem]

Description changed:

--- 
+++ 
@@ -7,3 +7,5 @@
 TypeError                                 Traceback (most recent call last)
 TypeError: 'NoneType' object is not iterable
 ```
+
+We fix this by properly initializing them with zero rays/lines instead of `None`.

[kliem]

Commit: 50f9232

[kliem]

Author: Laith Rastanawi, Jonathan Kliem

[LaisRast]

comment:3

I think it is good to go.
I will wait for the patchbot to finish so I can put it on "positive review".

[kliem]

comment:4

Bot is morally green.