Completing \overline, \sqrt, \hat... in LaTeX mode leaves the caret after an empty argument #3084

Environment

• MathLive 0.110.0
• macOS, Chrome/Electron

Steps to reproduce

1. Focus a mathfield.
2. Type \overline and press Space (or Enter) to complete it.
3. Type AB.

Expected

\overline{AB}: the caret goes into the argument, like it does for \frac.

Actual

\overline{} is inserted with the caret after it, so nothing is visible and AB lands outside. The same happens for \sqrt, \boxed, \hat, \vec, \underline. \frac and \binom get \placeholder{} arguments and work.

A missing argument parses to an empty body (scanArgument returns null, argAtoms(null) is []) instead of a placeholder, so complete()'s selectionMode: 'placeholder' has nothing to select.