Explorer node: Display current block instead of Treasury when using "block" with no arguments

[dbadol]

Current situation
When querying the explorer node with "block" and no argument, the result provided is the Treasury details. Although interesting per se, it does not make much sense to display that information by default (in particular now that the treasury allocation has ended, and moreover with that specific dataset that is a bit heavy to load).

Solution we propose
When querying the explorer node with "block" and no argument (no height nor kernel), the explorer node should return the details of the current block. That information is much more useful and makes more sense as default result.

Alternative considered
If we want the result of the query to be constant (and not changing every minute), we could have the explorer node return the Genesis block (instead of the Treasury), or better, the blockchain status (as for other queries with unknown arguments).