Fix the length of strings in ODBC vector operations
Saving a string of length N in the database saved it as a string of length N+1 with a trailing NUL explicitly written into the database and reading it back resulted in a string of length N+1 too, i.e. it didn't survive the round trip (notice that this bug only affected vector operations, scalar ones worked correctly). Fix this by not writing the trailing NUL in odbc_vector_use_type_backend and fix the strings returned from odbc_vector_into_type_backend by not assuming that they're NUL-terminated any more: in fact, they can be, but they also can be right-padded with spaces, which need to be removed. Notice that this does mean that we still have a problem with inconsistent behaviour for the strings with the trailing spaces between ODBC backend and others: ODBC one will strip them from the returned strings, but other backends will not. It is not clear what can be done about it, unfortunately.
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