memtx: raise an error if argument of select() is a "clear" dict

Currently, select/pairs/get look at an :array part of passed table,
that is why different problems occurs when passed to select() table
is not a regular array.

Now we will check if passed table is not "clear dict"
(not empty table without array part). Invalid queries
like s:select{key = 'value'} will fail with an appropriate
error, but the problems with dicts containing array part
still remain (for example, s:select{1, key = 'value'} is
the same as s:select{1}).

Closes #6167