change the way null values are handled in IPAC table writing

fixes #1344
add strip to remove extra whitespace in python3
add missing import statement
simplify new test to not to depend on votable and its unicode handling
remove try-except that could mask other errors
fix errors that appeard after rebasing
Merge pull request #1366 from hamogu/fix1344

Change the way null values are handled in IPAC table writing