refactor: optimize GetOrAdd method in signing_shares.h

Replaced the previous implementation of the GetOrAdd method with a more efficient approach using emplace. This change simplifies the logic by directly attempting to insert a new entry into the internal map, improving performance and readability while maintaining the same functionality.

Author: PastaPastaPasta

src/llmq/signing_shares.h

@@ -210,12 +210,9 @@ class SigShareMap
 
     T& GetOrAdd(const SigShareKey& k)
     {
-        T* v = Get(k);
-        if (!v) {
-            Add(k, T());
-            v = Get(k);
-        }
-        return *v;
+        auto& m = internalMap[k.first]; // Get or create outer map entry
+        auto result = m.emplace(k.second, T()); // Try to insert, returns pair<iterator, bool>
+        return result.first->second; // Return reference to the value (new or existing)
     }
 
     const T* GetFirst() const