db/version_set.cc: use !empty() instead of 'size() > 0'

Use empty() since it should be prefered as it has, following
the standard, a constant time complexity regardless of the
containter type. The same is not guaranteed for size().

Fix for:
[db/version_set.cc:2250]: (performance) Possible inefficient
 checking for 'column_families_not_found' emptiness.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

db/version_set.cc

@@ -2169,7 +2169,7 @@ Status VersionSet::Recover(
 
   // there were some column families in the MANIFEST that weren't specified
   // in the argument. This is OK in read_only mode
-  if (read_only == false && column_families_not_found.size() > 0) {
+  if (read_only == false && !column_families_not_found.empty()) {
     std::string list_of_not_found;
     for (const auto& cf : column_families_not_found) {
       list_of_not_found += ", " + cf.second;