`is_empty` returns true if `$string` is the [empty string].

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]}
}
======

Update: it seems that `string length $data` is even faster. I guess there is a recent optimisation in `string length` that avoids translating the binary to a string. See output below...


Test:

======
foreach cmd {
    {expr {$data == {}}}
    {expr {$data eq {}}}
    {string bytelength $data}
    {string equal $data {}}
    {string length $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


string length $data
  22 microseconds per iteration
  result = 191470592


is_empty $data
  42 microseconds per iteration
  result = 0

======

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