If there's no blocks in database (m_height == 0):
Don't assign incorrect block range to check.
Skip average block size check.
Test:
Run blockchain_converter with an existing source blockchain.bin and
a non-existent LMDB destination database.
The converter creates a BlockchainLMDB instance with zero height, due to
not being initialized with a genesis block, normally done by
Blockchain::init(). While different than the behavior of bitmonerod,
blockchain_import, and blockchain_export, the initialization hasn't been
strictly necessary.
The db batch size estimation normally uses an average block size, or a
default minimum block size, whichever is greater. In this case, as
there's no existing blocks to check for an average block size, the
default should be used.
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