db: return Miniheader with zero values rather than nil from FindLatestMiniheader when no headers are found

[kyriediculous]

What does this pull request do? Explain your changes. (required)
Instead of returning nil from db.FindLatestMiniHeader when no entries are found in the database we return a zero-value to avoid unexpected behaviour whereby no error and no value is returned.

This behaviour is similar to the underlying ethClient.HeaderByNumber implementation.

note
Consider changing blockWatch.Miniheader.Number to an int64 or big.Int instead of a pointer reference *big.Int to avoid explicit initialisation of this field.

&blockwatch.MiniHeader{
  Number: big.NewInt(0)
}

Specific updates (required)

• Added a test case
• Return zero value when no results found in FindLatestMiniHeader
• fixed other test cases for functions that use FindLatestMiniHeader
• Changed the check for blocksElapsed to be <=0 to avoid verbose logging when no blocks have elapsed.

How did you test each of these updates (required)
Ran unit tests

Does this pull request close any open issues?
Fixes #1803
Fixes #1780

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[yondonfu]

Looks like there are multiple places in block_watcher.go that depend on FindLatestMiniHeader() returning a nil header when there are no headers in the DB. Here is one and here is another. There may be more. Currently, I think returning the zero value for a header in FindLatestMiniHeader() might cause breakage in those areas.

Could be worth weighing continuing to return a nil header and just adding a check for that in the code path that was causing a nil pointer error.

[kyriediculous]

Reverted to old behaviour and added a nil check. Also added a quick one character fix for #1780.

Tested on rinkeby and checked if there were other cases we should add a nil check but didn't see any immeadiatly. Will double-check.

[yondonfu]

LGTM after squashing

[yondonfu]

One comment on changelog, but LGTM after fixing.

[kyriediculous]

This one okay to merge ?

[yondonfu]

Yep and already approved.