make GetFirstStoredBlock assert that 'start_block' always has data

And transfer the responsibility of verifying whether 'start_block'
has data or not to the caller.

This is because the 'GetFirstStoredBlock' function responsibility
is to return the first block containing data. And the current
implementation can return 'start_block' when it has no data!. Which
is misleading at least.

Edge case behavior change:
Previously, if the block tip lacked data but all preceding blocks
contained data, there was no prune violation. And now, such
scenario will result in a prune violation.

src/node/blockstorage.cpp

@@ -407,6 +407,7 @@ const CBlockIndex* BlockManager::GetFirstStoredBlock(const CBlockIndex& start_bl
 {
     AssertLockHeld(::cs_main);
     const CBlockIndex* last_block = &start_block;
+    assert(last_block->nStatus & BLOCK_HAVE_DATA);
     while (last_block->pprev && (last_block->pprev->nStatus & BLOCK_HAVE_DATA)) {
         last_block = last_block->pprev;
     }