Power Query - Replacing empty cells in source with null in M query

Hi,
I have a table in which say for example in  a table column title  has empty values for few rows .I am expanding my table in Power query and fetching only the title column.For few rows it is showing as error .I tried replacing errors with null however it worked but it is taking longer time . Is there anyway where I can check in if the table has empty rows then replace that with null else the values in title column for each row.

M query: 
let
Source = SharePoint.Tables("sharepoint site url",
#" " = Source{[Id=" "]}[Items],
#"Renamed Columns" = Table.RenameColumns(#" ",{{"ID", "ID.1"}}),
#"Removed Other Columns" = Table.SelectColumns(#"Renamed Columns",{"Id", "Title", "Modified", "Created"),
#"Expanded Author" = Table.ExpandRecordColumn(#"Removed Other Columns", "Author", {"Title"}, {"Author.Title"}),
#"Renamed Columns1" = Table.RenameColumns(#"Expanded Author",{{"Author.Title", "Author"}}),
#"Expanded Handler" = Table.ExpandRecordColumn(#"Renamed Columns1", "Handler", {"Title"}, {"Handler.Title"}),
#"Renamed Columns2" = Table.RenameColumns(#"Expanded Handler",{{"Handler.Title", "Handler"}}),
#"Expanded Assigned" = Table.ExpandRecordColumn(#"Renamed Columns2", "Assigned", {"Title"}, {"Assigned.Title"})
in
#"Expanded Assigned"

In the aboce m query the error is in the step #"Expanded Assigned" . I need the solution to belike if column title is blank then it should be null else it should give the value .So  during expanding the column I need to check on this condition.
Any suggestions please? 

Above is the error message and the error.