Unhandled NoneType exception

Summary

If you have an element with "depends:" but nothing in the depends, it gives a nonetype error

Steps to reproduce

make a normal element and add a "depends:" line to it, but no dependency beneath it

kind: stack

description: |

Main stack target for the bst build test

depends:

Attempt to build this project and you will get an exception

What is the current bug behavior?

A NoneType exception is thrown

What is the expected correct behavior?

I would expect an invalid YAML error

Relevant logs and/or screenshots

 Traceback (most recent call last):

 File "/home/phillipsmyth/.local/bin/bst", line 8, in <module>
    sys.exit(cli())

  File "/home/phillipsmyth/.local/lib/python3.5/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_frontend/cli.py", line 166, in override_main
    standalone_mode=standalone_mode, **extra)

  File "/home/phillipsmyth/.local/lib/python3.5/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)

  File "/home/phillipsmyth/.local/lib/python3.5/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))

  File "/home/phillipsmyth/.local/lib/python3.5/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)

  File "/home/phillipsmyth/.local/lib/python3.5/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)

  File "/home/phillipsmyth/.local/lib/python3.5/site-packages/click/decorators.py", line 27, in new_func
    return f(get_current_context().obj, *args, **kwargs)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_frontend/cli.py", line 336, in build
   build_all=all_)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_stream.py", line 182, in build
    dynamic_plan=True)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_stream.py", line 952, in _load
    fetch_subprojects=fetch_subprojects)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_pipeline.py", line 111, in load
    fetch_subprojects=fetch_subprojects)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_project.py", line 334, in load_elements
    fetch_subprojects=fetch_subprojects)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_loader/loader.py", line 134, in load
    loader._load_file(name, rewritable, ticker, fetch_subprojects, yaml_cache)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_loader/loader.py", line 278, in _load_file
    element = LoadElement(node, filename, self)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_loader/loadelement.py", line 78, in __init__
    self.deps = _extract_depends_from_node(self.node)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_loader/loadelement.py", line 135, in 
   _extract_depends_from_node
    depends = _extract_depends_from_node(node, key=Symbol.DEPENDS)

  File "/home/phillipsmyth/projects/buildstream/buildstream/_loader/loadelement.py", line 149, in _extract_depends_from_node
    for index, dep in enumerate(depends):

  TypeError: 'NoneType' object is not iterable

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

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