Fix issue when problem has an empty problem in pdlp

[Kh4ster]

Fixes #103

• Matrix with an empty problem no longer triggers an assert.
• If in PDLP mode, a LOG_INFO tells the user he should use DualSimplex instead and returns a NumericalError
• If in Concurrent mode, PDLP will stop but Dual Simplex will solve the problem

[rg20]

/merge

[chris-maes]

@Kh4ster This was already merged. But what about instead of just saying we can't handle a problem with no constraints in PDLP, why don't we just add an empty constraint (e.g 0 * x == 0, or -inf <= 0 * x <= inf )