small code improvements

Author: Barakudum

src/main/java/org/dynapi/squirtle/core/queries/JoinUsing.java

@@ -4,6 +4,7 @@
 import org.dynapi.squirtle.core.interfaces.SqlAbleConfig;
 import org.dynapi.squirtle.core.terms.criterion.Field;
 
+import java.util.ArrayList;
 import java.util.Collection;
 
 public class JoinUsing extends Join {
@@ -17,7 +18,7 @@ public JoinUsing(Selectable item, JoinType how, Collection<Field> fields) {
     @Override
     public JoinUsing replaceTable(Table currentTable, Table newTable) {
         item = (currentTable.equals(item)) ? newTable : item;
-        fields = fields.stream().map(field -> field.replaceTable(currentTable, newTable)).toList();
+        fields = new ArrayList<>(fields.stream().map(field -> field.replaceTable(currentTable, newTable)).toList());
         return this;
     }
 

src/main/java/org/dynapi/squirtle/core/terms/criterion/Field.java

@@ -31,7 +31,8 @@ public Field(String alias, String name, Selectable table) {
     public List<Node> nodes() {
         List<Node> nodes = new ArrayList<>();
         nodes.add(this);
-        nodes.addAll(table.nodes());
+        if (table != null)
+            nodes.addAll(table.nodes());
         return nodes;
     }
 

src/main/java/org/dynapi/squirtle/core/terms/criterion/Star.java

@@ -26,7 +26,8 @@ public Star(Selectable table) {
     public List<Node> nodes() {
         List<Node> nodes = new ArrayList<>();
         nodes.add(this);
-        nodes.addAll(table.nodes());
+        if (table != null)
+            nodes.addAll(table.nodes());
         return nodes;
     }