If ComplexCharToStr returns nil, set drawable = NO

This does not fix the issue of not checking whether a unicode code point
is actually drawable, but it avoids the following assertion:

    Assertion failed: (growBy > 0),
        function -[SharedCharacterRunData growAllocation:by:],
        file /Users/georgen/v2/SharedCharacterRunData.m,
        line 74.

This assertion fails when attaching to a previously detached tmux
session that has multiple horizontal and vertical splits with

    tmux -C attach

The assertion did not fail in all cases, but often enough to make
tmux integration unusable.