Ability to skip encrypting fields with null value

[adidaslevy]

I have encountered an issue that fields with null value are disappearing from document.
That is - the encryptFields function tries to encrypt null fields as if they are objects.

Additionally, encrypting null value generates a large string, which eventually wastes disk space.

See example of encrypted nulls:

"__enc_contactPerson" : true,
"__enc_contactPerson_d" : "31323334353637383930313233343536:fc52ec69cbf7d3c7ec8843605fb792fc", 
"__enc_name" : true,
"__enc_organization" : true,
"__enc_organization_d" : "31323334353637383930313233343536:fc52ec69cbf7d3c7ec8843605fb792fc" 
"__v" : 0,
"title" : null,
"__enc_birthDate" : true,
"__enc_birthDate_d" : "31323334353637383930313233343536:fc52ec69cbf7d3c7ec8843605fb792fc", 
"__enc_firstName" : true,
"__enc_formattedName" : true,
"__enc_lastName" : true,
"__enc_middleName" : true,
"__enc_middleName_d" : "",
"__enc_referenceID" : true,
"middleName": "31323334353637383930313233343536:ab45883e5f44441ed4a3ff5795fabd5935722aa9fb05e55a4af889efabe73f43"

where instead the document can be much cleaner:

"contactPerson" : null, 
"organization" : null 
"__v" : 0,
"title" : null,
"birthDate" : null, 
...

This Breaks the requirement of an encrypted field to be mandatory in a document on one hand, but on the other hand gives you much more flexibility.

Similarly to last time, I have created a fix with a test.