closes #2454
closes #8213
ref #7972

The gchar_cursor() == NUL check is already done in ins_ctrl_o.
ins_esc changes gchar_cursor() so this if block is probably never
entered.

Issue:

Pressing CTRL-O in insert mode at the end of the line and typing
:startinsert moves the cursor 1 column back, when I expect the cursor
to remain at the end of the line

This is a regression from Vim behavior. Since at least Vim version 7.0,
Vim returns you to insert mode at the end of the line.

091e7d033cbf0f4da068292ce4ac934f1c3dd91e is the first bad neovim commit

Steps to reproduce using `nvim -u NORC`:
`aaaa<C-o>:startinsert<CR>`

Fixes #6962

Default Vim behavior of i_CTRL-<Space> is to insert the last-inserted
text and exit insert mode. :help i_CTRL-@

Before this commit that did not happen because insert_handle_key()
checks for NUL instead of checking for ' ' with a CTRL `mod_mask`.

I'm leaving the check for NUL despite the fact that at the moment that
key is never seen when using the terminal UI (not for C-Space, nor C-@).
This is because I assume it's still allowed for other front-ends to pass
NUL, but at the moment the terminal UI isn't.