"missing text" is not displayed in translation set view

[yoavf]

Example:

This probably happens since 7b103dd - where the array of translations supplied by for_translation() is filtered with gp_is_not_empty_string()

Non translated entries will have an array with two null values. gp_is_not_empty_string returns true for them. A possible solution would be to amend it to

function gp_is_not_empty_string( $value ) {
    return is_string( $value ) && '' !== $value;
}

... but then the function name would not be semantically correct. Maybe we can rename it to gp_is_string_not_empty

[ocean90]

@yoavf See #560, instead of using a check for strings we can simply check if the value is null.