`is_empty` returns true if `$string` is `{}`.

It is efficient even if `$string` is large binary data (see test output below).

It would be nice if there was an optimisation in the interpreter for `... eq {}` or `string equal` to make this unnecessary.

======
proc is_empty {string} {
    expr {![binary scan $string c c]}
}

proc not_empty {string} {
    expr {![is_empty $string]}
}
======

Test:

======
foreach cmd {
    {expr {$data == {}}}
    {expr {$data eq {}}}
    {string bytelength $data}
    {string equal $data {}}
    {is_empty $data}
} {
    set data [file get ~/stuff/file.dat]
    puts "$cmd\n  [time {set r [eval $cmd]}]\n  result = $r\n"
    unset data
    puts ""
}
======

Output:

======
expr {$data == {}}
  1099855 microseconds per iteration
  result = 0


expr {$data eq {}}
  1094751 microseconds per iteration
  result = 0


string bytelength $data
  1076724 microseconds per iteration
  result = 378629364


string equal $data {}
  1078098 microseconds per iteration
  result = 0


is_empty $data
  42 microseconds per iteration
  result = 0

======