Ignore missing values for declared fields during sink to HBase

Sometimes not all values will be available for the declared fields, so the HBaseScheme has to allow for sinking even when there are missing values.
In my scenario, I have some records coming from a CSV file, but some columns are missing, which prevented me from sinking to HBase.
So, I added a "strict" flag that, when set to false, ignores null values (by default this flag is set true, to keep it backwards compatible).
This is similar to the "strict" flag used in Cascading's TextDelimited scheme.