bug#66073: 30.0.50; ERC 5.6: Improve handling of blank lines at ERC's prompt

[J.P.]

Changes from this bug attempted to fix a number of historical behavioral
quirks related to intercepting and validating prompt input preflight.
One thing they glossed over was not dealing with the confusing "No
process running" warning issued when a user submits input in an orphaned
target buffer. This unfortunate behavior has been with ERC for ages and
might well be cemented into the wider user "ecosystem" such that fixing
it interferes with things like a `condition-case' handler matching
against the exact wording, etc. However, given the annoying nature of
this style of input feedback, I think it makes sense to bite the bullet
here and just chance a minimally churn-inducing change. See attached.

0008-5.6-Clarify-warning-for-process-dependent-input-in-E.patch
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