确定点对象是否为空
BRISKPoints
|cornerPoints
|detectBRISKFeatures
|detectFASTFeatures
|detectHarrisFeatures
|detectKAZEFeatures
|detectMinEigenFeatures
|detectMSERFeatures
|detectORBFeatures
|detectSURFFeatures
|extractFeatures
|KAZEPoints
|matchFeatures
|MSERRegions
|ORBPoints
|SURFPoints