In #5664, `nim-json-rpc` dependency got bumped which included a change
in behaviour when processing `null` data for heap allocated objects.
- https://github.com/status-im/nim-json-rpc/pull/176
Old behaviour was to raise an exception, while new behaviour is to set
the value to `nil` but treat it as a successful parse. Old exceptions
were similar to "Parameter [result] expected JObject but got JNull".
As part of the `nim-json-rpc` bump in #5664, `el_manager.nim` was not
updated to match the new behaviour, leading to crash whenever its logic
assumes that a successfully parsed web3 `BlockObject` (heap allocated)
may be assumed to be non-`nil`.
As a quick remedy, the `el_manager.nim` is updated to transform `nil`
responses for `BlockObject` into `ValueError`, allowing reuse of the
existing and tested exception based processing.
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