Fix vertices(G) to return all vertices, including isolated ones

[benlorenz]

fixes #4426
cc: @YueRen, @bkholler

The tests in algebraic statistics all seem to pass even with this change, so I don't think we need vertices_with_positive_degree.

[lgoettgens]

The tests in algebraic statistics all seem to pass even with this change, so I don't think we need vertices_with_positive_degree.

IIUC, their graphs are all connected, and thus in particular have no isolated vertices.

[afkafkafk13]

The tests in algebraic statistics all seem to pass even with this change, so I don't think we need vertices_with_positive_degree.

IIUC, their graphs are all connected, and thus in particular have no isolated vertices.

Should there be one (small) testing case with an isolated vertex to prevent future regression? (Given that the authors did not come across the corner case by themselves, this might be a sensible idea.)

[benlorenz]

The tests in algebraic statistics all seem to pass even with this change, so I don't think we need vertices_with_positive_degree.

IIUC, their graphs are all connected, and thus in particular have no isolated vertices.

Should there be one (small) testing case with an isolated vertex to prevent future regression? (Given that the authors did not come across the corner case by themselves, this might be a sensible idea.)

This new testcase for vertices has many isolated vertices:

Oscar.jl/test/Combinatorics/Graph.jl

Lines 4 to 7 in 31c935c

	g = Graph{Directed}(5)
	@test n_vertices(g) == 5
	@test n_edges(g) == 0
	@test vertices(g) == 1:5

In Oscar 1.2.2:

julia> g = Graph{Directed}(5)
Directed graph with 5 nodes and no edges

julia> vertices(g)
ERROR: ArgumentError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer

[afkafkafk13]

This new testcase for vertices has many isolated vertices:

Oscar.jl/test/Combinatorics/Graph.jl

Lines 4 to 7 in 31c935c

	g = Graph{Directed}(5)
	@test n_vertices(g) == 5
	@test n_edges(g) == 0
	@test vertices(g) == 1:5

Thank you! I had overlooked that one. No further comments from me.