sim.nr_pq() fails to detect over-constrained buses when values are zero

[marcostfermin]

Describe the bug

nr_pq() detects over-constrained buses using truthiness checks like all((P_set[i], Q_set[i], V_set[i][0])). Valid values of 0.0 are treated as False, so buses that are still over-specified are not detected.

Reproduction Code

# Setup
import electricpy as ep

Ybus = [[-10j, 10j],
        [ 10j,-10j]]

V_set = [[1.0, 0.0], [None, None]]
P_set = [0.0, None]
Q_set = [0.0, None]

# Code which causes failure
ep.sim.nr_pq(Ybus, V_set, P_set, Q_set)

Expected behavior

The function should raise ValueError("Over-Constrained System") because P, Q, and V are all specified for the same bus, even if P and Q are zero.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. Linux]
• Python Version [e.g. 3.7]
• Version [e.g. 0.2.1]

Additional context

Checks should use explicit is not None logic instead of truthiness.