Add test for conic problem with constraints but no variables

[odow]

See this issue in ECOS: jump-dev/ECOS.jl#132 (comment)

We should see what happens for models of the form:

model = MOI.Utilities.Model{Float64}()
f = MOI.VectorAffineFunction(MOI.VectorAffineTerm{Float64}[], [1.0, 0.5, 0.5])
MOI.add_constraint(model, f, MOI.SecondOrderCone(3))

They might solve, or they might error, but they shouldn't segfault.

Test for MOI.OPTIMAL or MOI.INVALID_MODEL as the termination status.

[blegat]

What about tests that add empty equality constraints like test_linear_VectorAffineFunction_empty_row and test_conic_PositiveSemidefiniteConeTriangle ?
CSDP and SDPT3 fail these tests. Currently they throw an error.
Should they not throw any error but return INVALID_MODEL instead and the test would accept that ?
There is also the issue of tests having no constraint at all:
https://github.com/jump-dev/SDPT3.jl/blob/e9b3f5e713beb9222c6a8bbcfc9732368cdae29c/test/MOI_wrapper.jl#L85-L103
we should probably add at least one constraint when we test attributes.

[blegat]

DSDP also does not support problems with no constraints.