Modify Dax to hide if empty

I am using a time Dax measure taken from here: https://community.powerbi.com/t5/Community-Blog/Aggregating-Duration-Time/ba-p/22486#comments

It works wonderfully, but there is an issue I have encountered when using within a Pivot Table and then filtering. As I filter and the data set shrinks I find that users who should be filtered out as they are no longer in the selected date range still reflect as ::

This must be because the concatenate part of the expression therefore still shows as a value "::"

I tried creating a calculate formula to filter out the :: but it did not do anything, I'm new to Dax and I'm assuming my attempt was hamfisted.

Dur_No_Dot = CALCULATE(Chat[Duration],FILTER(chat,[Duration] <> "::")) 

So yes, pasted below is the code creating the formula, if anyone can tell me how to surpress the :: on a pivot table I'd much appreciate the help!

Duration = // Duration formatting 
// * @konstatinos 1/25/2016
// * Given a number of seconds, returns a format of "hh:mm:ss"
//
// We start with a duration in number of seconds
VAR Duration = Chat[chat_in_seconds]
// There are 3,600 seconds in an hour
VAR Hours =
    INT ( Duration / 3600)
// There are 60 seconds in a minute
VAR Minutes =
    INT ( MOD( Duration - ( Hours * 3600 ),3600 ) / 60)
// Remaining seconds are the remainder of the seconds divided by 60 after subtracting out the hours 
VAR Seconds =
    ROUNDUP(MOD ( MOD( Duration - ( Hours * 3600 ),3600 ), 60 ),0) // We round up here to get a whole number
// These intermediate variables ensure that we have leading zero's concatenated onto single digits
// Hours with leading zeros
VAR H =
    IF ( LEN ( Hours ) = 1, 
        CONCATENATE ( "0", Hours ),
        CONCATENATE ( "", Hours )
      )
// Minutes with leading zeros
VAR M =
    IF (
        LEN ( Minutes ) = 1,
        CONCATENATE ( "0", Minutes ),
        CONCATENATE ( "", Minutes )
    )
// Seconds with leading zeros
VAR S =
    IF (
        LEN ( Seconds ) = 1,
        CONCATENATE ( "0", Seconds ),
        CONCATENATE ( "", Seconds )
    )
// Now return hours, minutes and seconds with leading zeros in the proper format "hh:mm:ss"
RETURN
    CONCATENATE (
        H,
        CONCATENATE ( ":", CONCATENATE ( M, CONCATENATE ( ":", S ) ) )
    )