(PUP-3786) Correct text length calculation of [] and {}

The length of the source text associated with model elements is
important for error reporting and for the Puppet Strings documentation
tool. The grammar did not correctly produce the length of empty array
and hash expresses (they were always recorded with a length of 1).

This was caused by rules either referencing a non existant token, or by
only including the starting token in the calculation (which always leads
to a length of 1).

This commit changes the grammar's calls to associate length for the
erronous cases. Tests are also added.