The issue is reproducible on JDK7 b120, b119, b118, b100.

Please see at the following test:
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import java.security.Timestamp;
import java.security.cert.CertPath;
import java.security.cert.Certificate;
import java.security.cert.CertificateEncodingException;
import java.util.Collections;
import java.util.Date;
import java.util.Iterator;
import java.util.List;

public class TimestampTest {
    public static void main(String[] args) {
        Timestamp ts = new Timestamp(new Date(), new CertPathStub("type"));
        System.out.println("toString:" + ts.toString());
    }
}
class CertPathStub extends CertPath {
    public Iterator<String> getEncodings() {
        return null;
    }
    public byte[] getEncoded() throws CertificateEncodingException {
        return null;
    }
    public byte[] getEncoded(String encoding)
            throws CertificateEncodingException {
        return null;
    }
    public List<? extends Certificate> getCertificates() {
        return Collections.EMPTY_LIST;
    }
    protected CertPathStub(String type) {
        super(type);
    }
}
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It's output is:
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Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0
        at java.util.Collections$EmptyList.get(Collections.java:3205)
        at java.security.Timestamp.toString(Timestamp.java:145)
        at TimestampTest.main(TimestampTest.java:13)
-------------------------------------------------------

Since the spec for CertPath.getCertificates() states
"Returns: an immutable List of Certificates (may be empty, but not null)"
the implementation behaviour is erroneous.