Makes the encryption style default to substitution

src/main/java/net/knarcraft/bookswithoutborders/state/EncryptionStyle.java

@@ -5,7 +5,7 @@ package net.knarcraft.bookswithoutborders.state;
  */
 public enum EncryptionStyle {
     DNA("dna"),
-    SUBSTITUTION("");
+    SUBSTITUTION("substitution");
 
     private final String name;
 
@@ -25,6 +25,6 @@ public enum EncryptionStyle {
                 return style;
             }
         }
-        return null;
+        return SUBSTITUTION;
     }
 }