Fixed NPE when getContextClassLoader() returns null (#4193)

Author: fkorotkov

core/src/main/java/org/testcontainers/utility/MountableFile.java

@@ -130,6 +130,10 @@ private static URL getClasspathResource(@NotNull final String resourcePath, @Not
         classLoadersToSearch.add(MountableFile.class.getClassLoader());
 
         for (final ClassLoader classLoader : classLoadersToSearch) {
+            if (classLoader == null) {
+                continue;
+            }
+
             URL resource = classLoader.getResource(resourcePath);
             if (resource != null) {
                 return resource;