dbt docs generate errors when there's a "null" schema

[petehanssens]

Issue

Issue description

Fivetran inadvertently generated a null schema within our warehouse for google analytics data - this was subsequently fixed but due to laziness, I was yet to remove it.

When running dbt docs generate however this below error was produced.

Results

What happened? What did you expect to happen?

System information

The output of dbt --version:

    from svv_table_info
DEBUG:dbt:SQL status: SELECT in 2.90 seconds
DEBUG:dbt:On get_catalog_data: ROLLBACK
DEBUG:dbt:Flushing usage events
Encountered an error:
INFO:dbt:Encountered an error:
'NoneType' object has no attribute 'lower'
INFO:dbt:'NoneType' object has no attribute 'lower'
DEBUG:dbt:Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/dbt/main.py", line 72, in main
    results, succeeded = handle_and_check(args)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/dbt/main.py", line 117, in handle_and_check
    task, res = run_from_args(parsed)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/dbt/main.py", line 172, in run_from_args
    results = run_from_task(task, proj, parsed)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/dbt/main.py", line 180, in run_from_task
    result = task.run()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/dbt/task/generate.py", line 217, in run
    results = adapter.get_catalog(profile, self.project.cfg, manifest)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/dbt/adapters/default/impl.py", line 838, in get_catalog
    results = table.where(lambda r: r['table_schema'].lower() in schemas)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/agate/table/where.py", line 25, in where
    if test(row):
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/dbt/adapters/default/impl.py", line 838, in <lambda>
    results = table.where(lambda r: r['table_schema'].lower() in schemas)
AttributeError: 'NoneType' object has no attribute 'lower'

The operating system you're running on:

The python version you're using (probably the output of python --version)

Steps to reproduce

In as much detail as possible, please provide steps to reproduce the issue. Sample data that triggers the issue, example models, etc are all very helpful here.

Feature

Feature description

Making dbt docs generate more robust

Who will this benefit?

people who are strange enough to create null schemas or created for them in error by third parties.

[beckjake]

Fixed in #992