Added clauses to truncate null-values and integers correctly.

You get these with the postgres-adapter from boss_db.

Error messages are:
{function_clause,
    [{erlydtl_filters,truncatewords,
         [1800,8,[]],
         [{file,"src/erlydtl_filters.erl"},{line,994}]},
    ...

and
function_clause,
    [{erlydtl_filters,truncatewords,
         [null,8,[]],
         [{file,"src/erlydtl_filters.erl"},{line,994}]},
    ...