Print empty min/max strings in quotes (See #11472)

`bin/omero script params` was being misleading by showing
no value for min/max of String arguments when the min or
max was the empty string. This adds single quotes to the
empty string to make it clear that a min or max is present.
Fix flake8 warning
Merge pull request #2582 from joshmoore/11472-empty-min-max-string

11472 empty min max string