Fix ergoemacs-toggle-letter-case

If the cursor is on a delimiter such as '(' or ';' rather than a word,
`ergoemacs-toggle-letter-case' prints the message on the minibuffer:
"Wrong type argument: integer-or-marker-p". It might be confusing.

Perform case conversion only if both p1 and p2 are not nil.
Merge pull request #91 from aethanyc/fix-toggle-letter-case

Fix ergoemacs-toggle-letter-case