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When filling out number components that are not required, if left blank the number component throws notices when viewing the result:
Warning: number_format() expects parameter 1 to be double, string given in _webform_number_format() (line 704 of /Users/nate/Sites/drupal7/sites/all/modules/webform/components/number.inc).
This is because an empty string is not a number and should not be run through number_format().
Additionally, if a field is empty, we probably shouldn't be outputting the field prefix/suffix, because it doesn't make much sense to have a floating $ or kgs with no value next to it. This patch fixes these display problems with empty strings.