Rank of boundary_space() of modular symbols is wrong before boundary_map() is called

[pjbruin]

In SageMath 9.4.beta3, the rank of B changes from 0 to 3 in the following example:

sage: M = ModularSymbols(Gamma0(4), 4)
sage: B = M.boundary_space
sage: B.rank()
0
sage: M.boundary_map()
Hecke module morphism boundary map defined by the matrix
[-1  0  0]
[ 0 -1  0]
[ 0  0 -1]
Domain: Modular Symbols space of dimension 3 for Gamma_0(4) of weight ...
Codomain: Space of Boundary Modular Symbols for Congruence Subgroup Gamma0(4) ...
sage: B.rank()
3

Component: modular forms

Issue created by migration from https://trac.sagemath.org/ticket/32154

[pjbruin]

Description changed:

--- 
+++ 
@@ -1,8 +1,9 @@
-In SageMath 9.3, the dimension of the boundary space changes from 0 to 3 in the following example:
+In SageMath 9.4.beta3, the dimension of `B` changes from 0 to 3 in the following example:
 
 ```
 sage: M = ModularSymbols(Gamma0(4), 4)
-sage: M.boundary_space().dimension()
+sage: B = M.boundary_space
+sage: B.dimension()
 0
 sage: M.boundary_map()
 Hecke module morphism boundary map defined by the matrix
@@ -11,7 +12,7 @@
 [ 0  0 -1]
 Domain: Modular Symbols space of dimension 3 for Gamma_0(4) of weight ...
 Codomain: Space of Boundary Modular Symbols for Congruence Subgroup Gamma0(4) ...
-sage: M.boundary_space().dimension()
+sage: B.dimension()
 3
 ```
 

[pjbruin]

Description changed:

--- 
+++ 
@@ -1,9 +1,9 @@
-In SageMath 9.4.beta3, the dimension of `B` changes from 0 to 3 in the following example:
+In SageMath 9.4.beta3, the rank of `B` changes from 0 to 3 in the following example:
 
 ```
 sage: M = ModularSymbols(Gamma0(4), 4)
 sage: B = M.boundary_space
-sage: B.dimension()
+sage: B.rank()
 0
 sage: M.boundary_map()
 Hecke module morphism boundary map defined by the matrix
@@ -12,7 +12,7 @@
 [ 0  0 -1]
 Domain: Modular Symbols space of dimension 3 for Gamma_0(4) of weight ...
 Codomain: Space of Boundary Modular Symbols for Congruence Subgroup Gamma0(4) ...
-sage: B.dimension()
+sage: B.rank()
 3
 ```
 

[pjbruin]

comment:2

There is actually a warning about this in the docstring of the rank() method:

        .. warning::

           This number may change as more elements are coerced into
           this space!! (This is an implementation detail that will
           likely change.)