Merge branch 'autogen-empty-source-file-fix' into release-3.10

There is no corresponding merge request because this fix was
cherry-picked specifically for CMake 3.10.

Source/cmQtAutoGenerators.cxx

@@ -80,6 +80,7 @@ static bool ReadFile(std::string& content, std::string const& filename,
     std::size_t const length = cmSystemTools::FileLength(filename);
     cmsys::ifstream ifs(filename.c_str(), (std::ios::in | std::ios::binary));
     if (ifs) {
+      if (length > 0) {
         content.resize(length);
         ifs.read(&content.front(), content.size());
         if (ifs) {
@@ -90,6 +91,11 @@ static bool ReadFile(std::string& content, std::string const& filename,
             error->append("Reading from the file failed.");
           }
         }
+      } else {
+        // Readable but empty file
+        content.clear();
+        success = true;
+      }
     } else if (error != nullptr) {
       error->append("Opening the file for reading failed.");
     }