Adds checks to not normalize zero vectors

src/main/java/ch/njol/skript/expressions/ExprVectorLength.java

@@ -18,6 +18,7 @@
  */
 package ch.njol.skript.expressions;
 
+import ch.njol.util.VectorMath;
 import org.bukkit.event.Event;
 import org.bukkit.util.Vector;
 import org.eclipse.jdt.annotation.Nullable;
@@ -70,7 +71,7 @@ public void change(Event event, @Nullable Object[] delta, ChangeMode mode) {
 				//$FALL-THROUGH$
 			case ADD:
 				for (Vector vector : vectors) {
-					if (deltaLength < 0 && vector.lengthSquared() < deltaLength * deltaLength) {
+					if (VectorMath.isZero(vector) || (deltaLength < 0 && vector.lengthSquared() < deltaLength * deltaLength)) {
 						vector.zero();
 					} else {
 						double newLength = deltaLength + vector.length();
@@ -82,7 +83,7 @@ public void change(Event event, @Nullable Object[] delta, ChangeMode mode) {
 				break;
 			case SET:
 				for (Vector vector : vectors) {
-					if (deltaLength < 0) {
+					if (deltaLength < 0 || VectorMath.isZero(vector)) {
 						vector.zero();
 					} else {
 						if (!vector.isNormalized())

src/main/java/ch/njol/skript/expressions/ExprVectorNormalize.java

@@ -18,6 +18,7 @@
  */
 package ch.njol.skript.expressions;
 
+import ch.njol.util.VectorMath;
 import org.bukkit.event.Event;
 import org.bukkit.util.Vector;
 import org.eclipse.jdt.annotation.Nullable;
@@ -63,7 +64,7 @@ protected Vector[] get(Event event) {
 		if (vector == null)
 			return null;
 		vector = vector.clone();
-		if (!vector.isNormalized())
+		if (!VectorMath.isZero(vector) && !vector.isNormalized())
 			vector.normalize();
 		return CollectionUtils.array(vector);
 	}

src/main/java/ch/njol/util/VectorMath.java

@@ -198,4 +198,13 @@ public static void copyVector(Vector vector1, Vector vector2) {
 		vector1.setX(vector2.getX()).setY(vector2.getY()).setZ(vector2.getZ());
 	}
 
+	/**
+	 * Check whether or not each component of this vector is equal to 0.
+	 * <br>Replaces {@code Vector#isZero()} since that method was added in spigot 1.19.3
+	 * @return true if equal to zero, false if at least one component is non-zero
+	 */
+	public static boolean isZero(Vector vector) {
+		return (vector.getX() == 0 && vector.getY() == 0 && vector.getZ() == 0);
+	}
+
 }

src/test/skript/tests/regressions/6194-vectors + push.sk

@@ -0,0 +1,9 @@
+test "normalize zero vector":
+	set {_vector} to vector(0,0,0)
+	add 1 to vector length of {_vector}
+	assert {_vector} is vector(0,0,0) with "add to zero vector's length failed"
+
+	set vector length of {_vector} to 1
+	assert {_vector} is vector(0,0,0) with "set zero vector's length failed"
+
+	assert normalize {_vector} is vector(0,0,0) with "normalize zero vector failed"