Fix random seed calculation for a delegate when using previous block headers

[ManuGowda]

Expected behavior

Should return header for a delegate if random seed exists, if the random seed does not exists it should return undefined.

Actual behavior

With the genesis block height != 0, when selecting a random seed, it tries to look too much in the past (ie: height lower than genesis block) and fail

Steps to reproduce

• Enable debug logs
• Start Lisk Core 3.0.0-rc.0 node and sync with network
• During the synchronization observe the following error

18:28:15 INFO lisk-framework: Synchronization finished. (module=lisk:app)
{
 "lastBlockHeight": 14075361,
 "lastBlockID": "366ab5417d71dcba580952d218b18645e60771b68ae17934a63d039bd2a2d88d",
 "mechanism": "BlockSynchronizationMechanism"
}
18:28:15 WARN lisk-framework: Received invalid event message (module=lisk:app)
Message: Cannot read property 'generatorPublicKey' of undefined
 Trace: TypeError: Cannot read property 'generatorPublicKey' of undefined
    at findPreviousHeaderOfDelegate (/home/lisk/lisk-core/node_modules/lisk-framework/dist-node/modules/dpos/random_seed.js:33:27)

Which version(s) does this affect? (Environment, OS, etc...)

Lisk SDK 5.0.5, Lisk Core 3.0.0-rc.0