rpc: minize getTipHash() calls in gbt

Set tip at the start of the function and only update it for a long poll.

Additionally have getTipHash return an optional, so the
caller can explicitly check that a tip exists.

src/interfaces/mining.h

@@ -5,6 +5,7 @@
 #ifndef BITCOIN_INTERFACES_MINING_H
 #define BITCOIN_INTERFACES_MINING_H
 
+#include <optional>
 #include <uint256.h>
 
 namespace node {
@@ -29,8 +30,8 @@ public:
     //! If this chain is exclusively used for testing
     virtual bool isTestChain() = 0;
 
-    //! Returns the hash for the tip of this chain, 0 if none
-    virtual uint256 getTipHash() = 0;
+    //! Returns the hash for the tip of this chain
+    virtual std::optional<uint256> getTipHash() = 0;
 
    /**
      * Construct a new block template