Guard NetexId.createFrom against null valuePart

[Copilot]

NetexId.createFrom was silently converting a null valuePart to the literal string "null" via Java string concatenation, producing an invalid ID without any indication of the error.

Changes

• NetexId.createFrom: Added Objects.requireNonNull(valuePart, "valuePart") as a precondition check, failing fast with a NullPointerException instead of emitting a malformed ID.
• NetexIdTest: Added createFrom_whenNullValuePart_thenThrowNullPointerException to assert the null guard.

// Before: silently returned "XXX:FareContract:null"
NetexId.createFrom("XXX:FareContract:a", null);

// After: throws NullPointerException immediately
NetexId.createFrom("XXX:FareContract:a", null); // → NullPointerException

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