Create code to group a list of Contacts by their mailing state. The state keys should be uppercase, case-insensitive matching, and exclude contacts with null states.

Examples:

Input: contacts = [{FirstName: "John", LastName: "Doe", MailingState: "CA"},
    {FirstName: "Jane", LastName: "Smith", MailingState: "ca"},
    {FirstName: "Bob", LastName: "Johnson", MailingState: "NY"}]
Output: {"CA" => ["John Doe", "Jane Smith"],
    "NY" => ["Bob Johnson"]}

Input: contacts = [{FirstName: "John", LastName: "Doe", MailingState: null},
    {FirstName: "Jane", LastName: "Smith", MailingState: "TX"}]
Output: {"TX" => ["Jane Smith"]}