LibJS: Avoid unnecessary HashMap growth in Interpreter::enter_scope()

Don't bother pre-allocating a hash map if we're not gonna put anything
into it anyway.

Userland/Libraries/LibJS/Interpreter.cpp

@@ -113,7 +113,9 @@ void Interpreter::enter_scope(const ScopeNode& scope_node, ScopeType scope_type,
     }
 
     HashMap<FlyString, Variable> scope_variables_with_declaration_kind;
-    scope_variables_with_declaration_kind.ensure_capacity(16);
+
+    if (!scope_node.variables().is_empty())
+        scope_variables_with_declaration_kind.ensure_capacity(16);
 
     bool is_program_node = is<Program>(scope_node);