Merge pull request #23108 from crackwitz:issue-23107

Usage of imread(): magic number 0, unchecked result

* docs: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()

* samples, apps: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()

* tests: rewrite 0/1 to IMREAD_GRAYSCALE/IMREAD_COLOR in imread()

* doc/py_tutorials: check imread() result

samples/cpp/pca.cpp

@@ -59,7 +59,7 @@ static void read_imgList(const string& filename, vector<Mat>& images) {
     }
     string line;
     while (getline(file, line)) {
-        images.push_back(imread(line, 0));
+        images.push_back(imread(line, IMREAD_GRAYSCALE));
     }
 }