The condition for displaying the first char of the placeholder text in
the block cursor was `cursor_column == 0`. This meant that it was
displayed on the first column even when the placeholder text is not
being displayed. Instead this now shows it only when
`snapshot.is_empty()` - the same condition used to determine whether to
show placeholder text.
In the case of vim mode + agent panel message editor, this meant that if
you did `shift-enter` to make a newline and then `escape` to enter
normal mode, the block cursor would show `M` inside it as that's the
first character of the placeholder text "Message the agent - @ to
include context"
Release Notes:
- N/A
Michael Sloan
c0717bc613