containers: fix undefined behaviour when flatVector2D is empty but has items

Author: DMurataj01

src/containers/flatVector2D.tpp

@@ -1091,6 +1091,10 @@ const T* FlatVector2D<T>::operator[](std::size_t i) const
 {
     assert(isIndexValid(i));
 
+    if (m_v.empty()) {
+        return nullptr;
+    }
+
     std::size_t index = m_index[i];
     return &m_v[index];
 }
@@ -1106,6 +1110,10 @@ T* FlatVector2D<T>::operator[](std::size_t i)
 {
     assert(isIndexValid(i));
 
+    if (m_v.empty()) {
+        return nullptr;
+    }
+
     std::size_t index = m_index[i];
     return &m_v[index];
 }