[SPARK-12346][ML] Missing attribute names in GLM for vector-type features

Currently `summary()` fails on a GLM model fitted over a vector feature missing ML attrs, since the output feature attrs will also have no name. We can avoid this situation by forcing `VectorAssembler` to make up suitable names when inputs are missing names.

cc mengxr

Author: Eric Liang <ekl@databricks.com>

Closes #10323 from ericl/spark-12346.