Update fixed wrong import of @NotNull

src/main/java/cz/cvut/fit/tjv/seidlpet/semestralka/data/entity/Student.java

 package cz.cvut.fit.tjv.seidlpet.semestralka.data.entity;
-import com.sun.istack.NotNull;
-
 import javax.persistence.Entity;
 import javax.persistence.GeneratedValue;
 import javax.persistence.GenerationType;
 import javax.persistence.Id;
+import javax.validation.constraints.NotNull;
 import java.util.Objects;
 @Entity

src/main/java/cz/cvut/fit/tjv/seidlpet/semestralka/data/entity/Subject.java

 package cz.cvut.fit.tjv.seidlpet.semestralka.data.entity;
-import com.sun.istack.NotNull;
-
 import javax.persistence.*;
+import javax.validation.constraints.NotNull;
 import java.util.List;
 import java.util.Objects;

src/main/java/cz/cvut/fit/tjv/seidlpet/semestralka/data/entity/Teacher.java

 package cz.cvut.fit.tjv.seidlpet.semestralka.data.entity;
-import com.sun.istack.NotNull;
-
 import javax.persistence.Entity;
 import javax.persistence.GeneratedValue;
 import javax.persistence.GenerationType;
+import javax.validation.constraints.NotNull;
 import javax.persistence.Id;
 import java.util.Objects;