Return an empty optional in case there are no values

Otherwise we run into a read out of bounds in case the parameter is
stored, but with an empty vector value. E.g. from setting it with a
vector that comes from an external source which is empty.

include/podio/GenericParameters.h

@@ -219,6 +219,9 @@ std::optional<T> GenericParameters::get(const std::string& key) const {
     return it->second;
   } else {
     const auto& iv = it->second;
+    if (iv.empty()) {
+      return std::nullopt;
+    }
     return iv[0];
   }
 }