chore: RCTConvert - allocate array with capacity (facebook#35490)

Summary:
this is a trivial change that allocates the NSMutableArray with the correct capacity before it is filled

## Changelog

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[Internal] [Changed] - RCTConvertVecToArray - allocate array with capacity

Pull Request resolved: facebook#35490

Test Plan: tested locally: the code builds and runs as expected

Reviewed By: cipolleschi

Differential Revision: D41548050

Pulled By: GijsWeterings

fbshipit-source-id: a5b947331d6c5fffcfecc7c20c827f42442b1ab8

Libraries/TypeSafety/RCTConvertHelpers.h

@@ -23,7 +23,7 @@ using LazyVector = FB::LazyVector<T, id>;
 template <typename ContainerT>
 NSArray *RCTConvertVecToArray(const ContainerT &vec, id (^convertor)(typename ContainerT::value_type element))
 {
-  NSMutableArray *array = [NSMutableArray new];
+  NSMutableArray *array = [[NSMutableArray alloc] initWithCapacity:vec.size()];
   for (size_t i = 0, size = vec.size(); i < size; ++i) {
     id object = convertor(vec[i]);
     array[i] = object ?: (id)kCFNull;