Let VRT::Map#find_node accept nil values

[adamrdavid]

Description

Make VRT::Map#find_node more durable to unexpected input

• Resolves: VRT::Map#find_node hits uninformative error when passed bad input #32
• Also adds some much needed spec coverage around VRT::Map#find_node

Checklist:

• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have added entries to CHANGELOG.md
• I have not incremented version.rb

[maschwenk]

@adamrdavid what do you think about putting this check in walk_tree instead?

[adamrdavid]

@maschwenk the error that get's raised when string is nil comes from string + max_depth

I have updated the readme to better describe the situation.

[maschwenk]

@adamrdavid I guess it makes no sense to try to cache invalid vrt_ids so this is ok

[adamrdavid]

At any rate this needs to be added to get_lineage since that exhibits the same behavior of calling input + thing. If you think of a better place to put the check let me know.