[#34] VistA Functions: handle SQL NULLs (missed in previous commit)

A couple of things missed in the previous commit:

• Octo sends EMPTY strings from Octo if specified as literals as SQL NULLs. That means that REPLACE('AB','B','') did not work because the third parameter was not an empty string as expected by the M function.
• To be safe, FMGET (which is always supposed to be used with literals for the first and second arguments) will also protect itself if the first two arguments are passed as empty strings.
• Ditto for second argument of DATETIME.
• The rest of the functions were reviewed and no changes were thought to be currently needed.
• In addition, DATETIME function is supposed to accept one or two arguements. This was missed in the last commit, and a one argument form has been created; and tests now added to TVD0011 and TVD0012 tests in the Octo repo.