Have createNewBlock() wait for a tip

- return null on shutdown instead of the last tip
- ignore timeout value node initialization

This allows consumers of BlockTemplate to safely
assume that a tip is connected, instead of having
to account for startup and early shutdown scenarios.

src/interfaces/mining.h

@@ -101,10 +101,13 @@ public:
     virtual std::optional<BlockRef> waitTipChanged(uint256 current_tip, MillisecondsDouble timeout = MillisecondsDouble::max()) = 0;
 
    /**
-     * Construct a new block template
+     * Construct a new block template.
+     *
+     * During node initialization, this will wait until the tip is connected.
      *
      * @param[in] options options for creating the block
-     * @returns a block template
+     * @retval BlockTemplate a block template.
+     * @retval std::nullptr if the node is shut down.
      */
     virtual std::unique_ptr<BlockTemplate> createNewBlock(const node::BlockCreateOptions& options = {}) = 0;