Solve: If the symbol don't depends of the system don't is returned

[latot]

Hi, basically in automated process can happen we request a symbol but don't exist in the system, in that case the symbol should return the UniversalSet(), if the system don't depends of the symbol it can takes any value.

>>> solve([Eq(x+y,1), Eq(x-y,1)], [x, y, z])
{x: 1, y: 0}

to

>>> solve([Eq(x+y,1), Eq(x-y,1)], [x, y, z])
{x: 1, y: 0, z: UniversalSet()}

This is very important, can be confuse the solutions with #11658

Thx. Cya.