Fix cannot generate network watchlist report when database is empty

                    System.currentTimeMillis());

            final WatchlistReportDbHelper.AggregatedResult aggregatedResult =

                    mDbHelper.getAggregatedRecords();

            if (aggregatedResult == null) {

                Slog.i(TAG, "Cannot get result from database");

                return;

            }

            // Get all digests for watchlist report, it should include all installed

            // application digests and previously recorded app digests.

            final List<String> digestsForReport = getAllDigestsForReport(aggregatedResult);

package com.android.server.net.watchlist;

import android.annotation.Nullable;

import android.content.ContentValues;

import android.content.Context;

import android.database.Cursor;

        final Set<String> appDigestList;

        // The c&c domain or ip visited before.

        final String cncDomainVisited;

        @Nullable final String cncDomainVisited;

        // A list of app digests and c&c domain visited.

        final HashMap<String, String> appDigestCNCList;

                    WhiteListReportContract.TABLE, DIGEST_DOMAIN_PROJECTION, selectStatement,

                    new String[]{"" + twoDaysBefore, "" + yesterday}, null, null,

                    null, null);

            if (c == null || c.getCount() == 0) {

            if (c == null) {

                return null;

            }

            final HashSet<String> appDigestList = new HashSet<>();