Add validation for required fields

src/main/java/es/uvigo/esei/daa/entities/Pet.java

 package es.uvigo.esei.daa.entities;
 
+import static java.util.Objects.requireNonNull;
+
 public class Pet {
 	
     private int id;
@@ -31,7 +33,7 @@ public class Pet {
 	}
 	
 	public void setName(String name) {
-		this.name = name;
+		this.name = requireNonNull(name, "Name can't be null");;
 	}
 	
 	public String getKind() {
@@ -39,7 +41,7 @@ public class Pet {
 	}
 	
 	public void setKind(String kind) {
-		this.kind = kind;
+		this.kind = requireNonNull(kind, "Kind can't be null");;
 	}
 	
 	public String getBreed() {
@@ -47,7 +49,7 @@ public class Pet {
 	}
 	
 	public void setBreed(String breed) {
-		this.breed = breed;
+		this.breed = requireNonNull(breed, "Breed can't be null");;
 	}
 	
 	public int getOwner() {