How IllegalArgumentException automatically handled inside 'if' condition

I wanted to update the lastModifiedDate to current date and perform some action, only if the current date is greater than lastModifiedDate.

But I was concerned about getting NullPointerException near "dateToday.isAfter(lastModifiedDate.get("lastUpdated"))" inside if condition, when executed at the first time because, there is no key-value pair for "lastUpdated" key, yet in the HashMap. Below is the code.

import java.util.HashMap;
import org.joda.time.DateTimeZone;
import org.joda.time.LocalDate;

public class TestDate {

    static HashMap<String, LocalDate> lastModifiedDate = new HashMap<>();
    static int unityResponsesCount;

    public void resetUnityResponsesCount() {
        unityResponsesCount = 0;
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        LocalDate dateToday = LocalDate.now(DateTimeZone.getDefault());

        if (lastModifiedDate.isEmpty() || dateToday.isAfter(lastModifiedDate.get("lastUpdated"))) {
            lastModifiedDate.put("lastUpdated", dateToday);
            TestDate testDate = new TestDate();
            testDate.resetUnityResponsesCount();
        }
    }

}

Before running, I debugged the code and when I inspect only "dateToday.isAfter(lastModifiedDate.get("lastUpdated"))", I got,

java.lang.IllegalArgumentException: Partial cannot be null at org.joda.time.base.AbstractPartial.isAfter(AbstractPartial.java:351) at org.theorem.scan.qa.util.TestDate.main(TestDate.java:25)

But if I inspect complete if statement, lastModifiedDate.isEmpty() || dateToday.isAfter(lastModifiedDate.get("lastUpdated")), I'm getting "TRUE". If this IF statement yielded Exception, I should have provided separate if conditions with the same assignment inside each of them, lastModifiedDate.put("lastUpdated", dateToday);.

I'm just astonished, how IllegalArgumentException is handled automatically inside if condition.