bug: fix NullPointerException in optional fields

Source.getEmail() returns a null, using toString() on a null causes the Exception.
Since Optional.ofNullable only handles the null cases within its parameters, to handle null pointer safely, convert it to string outside of ofNullable()
Remove Optional Values in Person.equals() to avoid null pointer

src/main/java/seedu/address/model/person/Person.java

@@ -241,7 +241,6 @@ public boolean equals(Object other) {
                 && phone.equals(otherPerson.phone)
                 && address.equals(otherPerson.address)
                 && dateOfBirth.equals(otherPerson.dateOfBirth)
-                && dateOfAdmission.equals(otherPerson.dateOfAdmission)
                 && sex.equals(otherPerson.sex);
     }
 

src/main/java/seedu/address/storage/JsonAdaptedPerson.java

@@ -85,14 +85,14 @@ public JsonAdaptedPerson(Person source) {
         this.dateOfBirth = source.getDateOfBirth().toString();
         this.sex = source.getSex().toString();
         this.status = source.getStatus().toString();
-        this.email = Optional.ofNullable(source.getEmail().toString());
-        this.country = Optional.ofNullable(source.getCountry().toString());
-        this.allergies = Optional.ofNullable(source.getAllergies().toString());
-        this.bloodType = Optional.ofNullable(source.getBloodType().toString());
-        this.condition = Optional.ofNullable(source.getCondition().toString());
-        this.dateOfAdmission = Optional.ofNullable(source.getDateOfAdmission().toString());
-        this.diagnosis = Optional.ofNullable(source.getDiagnosis().toString());
-        this.symptom = Optional.ofNullable(source.getSymptom().toString());
+        this.email = Optional.ofNullable(source.getEmail()).map(Email::toString);
+        this.country = Optional.ofNullable(source.getCountry()).map(Country::toString);
+        this.allergies = Optional.ofNullable(source.getAllergies()).map(Allergies::toString);
+        this.bloodType = Optional.ofNullable(source.getBloodType()).map(BloodType::toString);
+        this.condition = Optional.ofNullable(source.getCondition()).map(Condition::toString);
+        this.dateOfAdmission = Optional.ofNullable(source.getDateOfAdmission()).map(DateOfAdmission::toString);
+        this.diagnosis = Optional.ofNullable(source.getDiagnosis()).map(Diagnosis::toString);
+        this.symptom = Optional.ofNullable(source.getSymptom()).map(Symptom::toString);
     }
 
     /**