Improve erc-warn-about-blank-lines behavior

Improve erc-warn-about-blank-lines behavior

* etc/ERC-NEWS: Mention more detailed feedback when option
`erc-warn-about-blank-lines' is non-nil.
* lisp/erc/erc-common.el (erc--input-split): Add `abortp' slot to
allow a premature exit while validating prompt input.
* lisp/erc/erc.el (erc-warn-about-blank-lines): Clarify meaning of
"blank lines" in doc string, and mention interaction with
companion option `erc-send-whitespace-lines'.
(erc-inhibit-multiline-input): Fix inaccurate description in doc
string.
(erc--input-review-functions): Move
`erc--discard-trailing-multiline-nulls' to end of list, after
`erc--run-input-validation-checks' so that the latter doesn't have to
resplit the original input string in order to issue feedback.
(erc--blank-in-multiline-input-p): Remove unused internal function
originally slated to be part of ERC 5.6.
(erc--check-prompt-input-for-something): New trivial validation
function to check if the input is empty.
(erc--count-blank-lines): New function that tallies up the number of
blank and whitespace-only lines in the current input.  One downside of
this design is that this function's conclusions aren't shared with
`erc--discard-trailing-multiline-nulls', which must decide on its own
how many lines to strip.
(erc--check-prompt-explanation): New variable.
(erc--check-prompt-input-for-multiline-blanks): Rework significantly
to provide more informative messages and more sensible behavior for
common cases with respect to relevant option values.
(erc--check-prompt-input-functions): Add new validation function
`erc--check-prompt-for-something'.
(erc--run-input-validation-checks): Set `abortp' slot of
`erc--input-split' when hooks return a non-string, rather than
generate an unhelpful fallback message.  Also print a message when
the variable `erc--check-prompt-explanation' is non-nil.
(erc-send-current-line): When the `abortp' slot of the current
`erc--input-split' object is non-nil, forgo normal input processing.
This fixes a regression in 5.6-git, which previously emitted an error
message when it should have just exited the command.
(erc--discard-trailing-multiline-nulls): Always run, regardless of
`erc-send-whitespace-lines', and leave a blank line behind when
stripping a message consisting of only blank lines.
(erc--run-send-hooks): Always run hooks and surrounding logic rather
than only when hooks are populated.
* test/lisp/erc/erc-tests.el (erc--refresh-prompt): Add assertion and
use default value for `erc--input-review-functions'.
(erc-ring-previous-command): Use default value for
`erc--input-review-functions' and ensure `erc-current-nick' does not
error.
(erc--blank-in-multiline-input-p): Remove obsolete test.
(erc--check-prompt-input-functions): Update expected message.
(erc--discard-trailing-multiline-nulls, erc--count-blank-lines): New
tests.
(erc-tests--check-prompt-input--expect,
erc-tests--check-prompt-input-messages): New helper variables.
(erc--check-prompt-input-for-multiline-blanks,
erc--check-prompt-input-for-multiline-blanks/explanations): New tests.
(erc-send-whitespace-lines): Expect hook-processing logic to pad empty
lines instead of deferring to `erc-send-input-line-function'.
(Bug#66073)