Make null hash constants constexpr

Simplify m_template initialization in miner

src/crypto/hash.h

@@ -89,8 +89,8 @@ namespace crypto {
     epee::to_hex::formatted(o, epee::as_byte_span(v)); return o;
   }
 
-  const static crypto::hash null_hash = {};
+  constexpr static crypto::hash null_hash = {};
-  const static crypto::hash8 null_hash8 = {};
+  constexpr static crypto::hash8 null_hash8 = {};
 }
 
 CRYPTO_MAKE_HASHABLE(hash)

src/cryptonote_basic/miner.cpp

@@ -99,7 +99,7 @@ namespace cryptonote
 
 
   miner::miner(i_miner_handler* phandler):m_stop(1),
-    m_template(block{}),
+    m_template{},
     m_template_no(0),
     m_diffic(0),
     m_thread_index(0),