Look up a param as a trimmed non-empty string, or null.
Returns null for whitespace as well as for absent, so a required-field check treats ' ' the way the caller meant it rather than passing a blank list name into a label lookup.
' '
Look up a param as a trimmed non-empty string, or null.
Returns null for whitespace as well as for absent, so a required-field check treats
' 'the way the caller meant it rather than passing a blank list name into a label lookup.