fix(utils): guard null in checkAndFixMas + removeTrailingZeroes

[gpitel]

Summary

Two pre-existing null-handling gaps in assets/js/utils.js, both surfaced by real designs:

• checkAndFixMas — a just-added winding can leave a null excitation in excitationsPerWinding. The signal loop then dereferences exc[signal] and throws, which crashed WireInfo / BasicCoreSelector on mount and made the wire-configuration step unusable. Skip null entries.
• removeTrailingZeroes — null/undefined slip past isNaN()/isFinite(), then value.toFixed() throws. This crashed WaveformOutput rendering, so the Operating Point view failed to render. Return early for null.

Change

-    if (isNaN(value) || !isFinite(value)) {
+    if (value == null || isNaN(value) || !isFinite(value)) {

   for (const exc of op.excitationsPerWinding) {
+      if (exc == null) continue;
       for (const signal of ['current', 'voltage']) {

Minimal, defensive, no behavior change for valid inputs. +2/−1.

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