[core] Skip zero-initialization of StaticVector data array

Remove value-initialization ({}) from StaticVector's underlying
std::array. Only elements [0, count_) are ever accessed, so
initializing the full array is wasted work.

This eliminates a memset on every construction. Most impactful for
stack-allocated StaticVectors in hot paths like
APIPlaintextFrameHelper::write_protobuf_messages, which was
memsetting 276 bytes of iovec storage on every BLE proxy flush.

esphome/core/helpers.h

@@ -215,7 +215,7 @@ template<typename T, size_t N> class StaticVector {
   using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
  private:
-  std::array<T, N> data_{};
+  std::array<T, N> data_;  // intentionally not value-initialized to avoid memset
   size_t count_{0};
 
  public: