`man` consults its own configured search path only when MANPATH holds an empty entry;
a list without one replaces the default outright.
`nvm use` produced exactly that,
so activating a version could hide every system man page.
Contribute the empty entry as a trailing one,
so that nvm's directory keeps precedence over the system's,
and only when the list has none already,
so that repeated `nvm use` calls are idempotent and an empty entry the user placed stays where they put it.
The `$(manpath)` snapshot this replaces went stale the moment man's configuration changed,
and cost a subprocess per `nvm use`.
It was also dead: `local MANPATH` scoped the assignment to `nvm()`,
so `export` did not outlive the call and MANPATH went untouched whenever it started out unset.
Only ksh, where `local` is not a builtin, ever ran it.
`nvm deactivate` now unsets MANPATH where nvm's was the only real entry,
rather than leaving a bare `:` behind.
Reported and diagnosed by @al0ksar in #3890, which used a leading empty entry instead.
That one accrues one more colon on every `nvm use`,
which `nvm deactivate` then leaves behind,
and it loses to the system's man pages wherever nvm's bin directory is not first on PATH.
Supersedes #2077.
Refs #3890
Refs #2077
nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.
Jordan Harband
Xandor Schiefer