Fix NPE in CallLogCacheLollipopMr1.isVoicemailNumber()

package com.android.dialer.app.calllog.calllogcache;

import android.content.Context;

import android.support.annotation.Nullable;

import android.telecom.PhoneAccountHandle;

import com.android.dialer.app.calllog.CallLogAdapter;

import com.android.dialer.util.CallUtil;

   * Returns true if the given number is the number of the configured voicemail. To be able to

   * mock-out this, it is not a static method.

   */

  public abstract boolean isVoicemailNumber(PhoneAccountHandle accountHandle, CharSequence number);

  public abstract boolean isVoicemailNumber(

      PhoneAccountHandle accountHandle, @Nullable CharSequence number);

  /**

   * Returns {@code true} when the current sim supports checking video calling capabilities via the

package com.android.dialer.app.calllog.calllogcache;

import android.content.Context;

import android.support.annotation.Nullable;

import android.telecom.PhoneAccountHandle;

import android.text.TextUtils;

import android.util.ArrayMap;

import com.android.dialer.calllogutils.PhoneAccountUtils;

import com.android.dialer.telecom.TelecomUtil;

  }

  @Override

  public boolean isVoicemailNumber(PhoneAccountHandle accountHandle, CharSequence number) {

  public boolean isVoicemailNumber(

      PhoneAccountHandle accountHandle, @Nullable CharSequence number) {

    if (TextUtils.isEmpty(number)) {

      return false;

    }

    return TelecomUtil.isVoicemailNumber(mContext, accountHandle, number.toString());

  }