Use length of children returned from RenderTree.childiter

[salkinium]

This is just a small fix, but it allows to use the childiter function to not just sort, but also filter nodes when rendering.
I need to "hide" some node types when rendering, and thus do this in childiter:

    def childiter(items):
        items = [i for i in items if i.type in lbuild.format.SHOW_NODES]
        return sorted(items, key=lambda item: (item.type, item.name))

However, this renders an additional node line without the node:

    ├── Module(modm:build)   Build System Generators
    │   ├── Module(modm:build:scons)   SCons Build Script Generator
    │   │   ╰── EnumerationOption(info.git) = Disabled in [Disabled, Info, Info+Status]
    ├── Module(modm:cmsis)   ARM CMSIS Support

After this fix:

    ├── Module(modm:build)   Build System Generators
    │   ╰── Module(modm:build:scons)   SCons Build Script Generator
    │       ╰── EnumerationOption(info.git) = Disabled in [Disabled, Info, Info+Status]
    ├── Module(modm:cmsis)   ARM CMSIS Support

cc @c0fec0de

[coveralls]

Coverage increased (+0.0002%) to 99.861% when pulling 71cfd0d on salkinium:fix/RenderTree.childiter into 775477e on c0fec0de:master.

[coveralls]

Coverage increased (+0.0002%) to 99.861% when pulling 71cfd0d on salkinium:fix/RenderTree.childiter into 775477e on c0fec0de:master.

[coveralls]

Coverage increased (+0.0002%) to 99.861% when pulling 71cfd0d on salkinium:fix/RenderTree.childiter into 775477e on c0fec0de:master.

[salkinium]

Ping @c0fec0de

[c0fec0de]

Will take care within the Next 3 days. Sorry for the delay Niklas Hauser <notifications@github.com> schrieb am Di. 23. Juli 2019 um 21:17:

…

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