Feature: check for spaces and "NA" in `validation_report()`

[martinctc]

Spaces and other text strings such as "NA", "N/A", and "#N/A" do not currently get detected as missing values by validation_report(). This is worth flagging to the analyst because they are effectively missing values.

A line that flags the presence of such values if they exist, e.g. "There are x values which may potentially represent missing values." May also be helpful to show what these values are.