Fix typing in balances-substrate-equilibrium

[chidg]

Describe the bug A lack of typing on one variable in SubEquilibriumModule allowed a major bug to occur The tokenBalances variable here is untyped, which means the value returned when this object is accessed by key here can be undefined, but it is typed as any.

For engineering: Test Please write a couple of unit test that cover the cases of balance queries containing defined and undefined free values.