Convert null dates to all dates existing into table

Try this in a new table:

New Table =
VAR _Dates =
    DISTINCT ( OriginalTable[Date] )
VAR _BlankVal =
    CALCULATETABLE (
        VALUES ( OriginalTable[User] ),
        FILTER ( OriginalTable, ISBLANK ( OriginalTable[Date] ) )
    )
VAR _AssignDates =
    FILTER ( CROSSJOIN ( _BlankVal, _Dates ), NOT ( ISBLANK ( [Date] ) ) )
VAR _OrigOK =
    FILTER ( OriginalTable, NOT ( ISBLANK ( OriginalTable[Date] ) ) )
RETURN
    UNION ( _OrigOK, _AssignDates )

Or this in Power Query (you will need to use your source table code for Source and Source1 lines)

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUTIw1AciIwMjQ6VYHaiQEYqQM0jIGEXICSikFBsLAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [User = _t, Date = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"User", type text}, {"Date", type date}}),
    #"GoodRows" = Table.SelectRows(#"Changed Type", each ([Date] <> null)),
    Source1 = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUTIw1AciIwMjQ6VYHaiQEYqQM0jIGEXICSikFBsLAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [User = _t, Date = _t]),
    #"Changed Type1" = Table.TransformColumnTypes(Source1,{{"User", type text}, {"Date", type date}}),
    #"Filtered Rows" = Table.SelectRows(#"Changed Type1", each ([Date] = null)),
    #"Removed Columns" = Table.RemoveColumns(#"Filtered Rows",{"Date"}),
    #"Added Custom" = Table.AddColumn(#"Removed Columns", "Custom", each #"GoodRows"),
    #"Expanded Custom" = Table.ExpandTableColumn(#"Added Custom", "Custom", {"Date"}, {"Custom.Date"}),
    #"Renamed Columns" = Table.RenameColumns(#"Expanded Custom",{{"Custom.Date", "Date"}}),
    #"Appended Query" = Table.Combine({#"GoodRows", #"Renamed Columns"}),
    #"Changed Type2" = Table.TransformColumnTypes(#"Appended Query",{{"Date", type date}})
    
in
    #"Changed Type2"