DocLookup has counter-intuitive behaviour

[jpountz]

DocLookup has several issues:

• containsKey only looks at the mappings regardless of the current document
• get throws an exception if the field is not mapped (should it return null instead?)
• get returns an empty list instead of null if the document has no values (while fielddata has the ability to know if a given field exists in a particular document)

[nik9000]

+1