[PP] Bug when editing yaml file to add version

[jtcohen6]

Describe the bug

1. Create a .yml file with nothing in it:

# models/schema.yml

2. dbt parse
3. Add version: 2 to the .yml file:

# models/schema.yml

version: 2

4. dbt --partial-parse parse

Error message:

Partial parsing enabled: 0 files deleted, 0 files added, 1 files changed.
Encountered an error:
'version'

Stracktrace

2021-07-13 21:40:27.083754 (MainThread): Traceback (most recent call last):
  File "/usr/local/Cellar/dbt/0.20.0_1/libexec/lib/python3.8/site-packages/dbt/main.py", line 125, in main
    results, succeeded = handle_and_check(args)
  File "/usr/local/Cellar/dbt/0.20.0_1/libexec/lib/python3.8/site-packages/dbt/main.py", line 203, in handle_and_check
    task, res = run_from_args(parsed)
  File "/usr/local/Cellar/dbt/0.20.0_1/libexec/lib/python3.8/site-packages/dbt/main.py", line 256, in run_from_args
    results = task.run()
  File "/usr/local/Cellar/dbt/0.20.0_1/libexec/lib/python3.8/site-packages/dbt/task/parse.py", line 85, in run
    self.get_full_manifest()
  File "/usr/local/Cellar/dbt/0.20.0_1/libexec/lib/python3.8/site-packages/dbt/task/parse.py", line 64, in get_full_manifest
    manifest = loader.load()
  File "/usr/local/Cellar/dbt/0.20.0_1/libexec/lib/python3.8/site-packages/dbt/parser/manifest.py", line 212, in load
    project_parser_files = partial_parsing.get_parsing_files()
  File "/usr/local/Cellar/dbt/0.20.0_1/libexec/lib/python3.8/site-packages/dbt/parser/partial.py", line 125, in get_parsing_files
    self.change_schema_file(file_id)
  File "/usr/local/Cellar/dbt/0.20.0_1/libexec/lib/python3.8/site-packages/dbt/parser/partial.py", line 429, in change_schema_file
    saved_schema_file.pp_dict = {"version": saved_yaml_dict['version']}
KeyError: 'version'

The output of dbt --version:

$ dbt --version
installed version: 0.20.0
   latest version: 0.20.0

Up to date!

[leahwicz]

@gshank this might be a partial parsing bug. Could you please take a look?

[gshank]

The partial parsing code assumes that a SchemaSourceFile from the partial parse file has already been validated and so would have the 'version' key. Should be easy to fix.