More consistent null handing

[xvrl]

• return null for missing schema columns (e.g. for groupBy)
• treat empty string and null the same, but always return null
• storage adapter uses empty string internally for filters / lookups
• allow specifying null in filters
• for multi-value dimensions, if the dimension has multiple values:
  1. dim extraction returns null -> null
  2. missing column -> null
  3. [] -> []
  4. [""] -> [null]
  5. ["", "a"] -> [null, "a"]
• for single value dimensions
  1. missing column -> null
  2. dim extraction returns null -> null
  3. [""] -> null
  4. [] -> null