Bug 1773802 - Ignore empty strings when spellchecking; r=smaug, a=dmeehan

This adds a check to see if the encoded word is not empty and does not start
with the null character. Hunspell accepts C-style strings and marks the empty
string as correctly spelled. This prevents other dictionaries from detecting
misspelled strings in languages that use a different charset, which has lead
to problems when using the en-US dictionary packaged with Firefox and Greek,
Hebrew and Russian dictionaries, where misspellings are not detected in the
non-English language.

Differential Revision: https://phabricator.services.mozilla.com/D149899