T-SQL: Stored Procedure - variable assignment

Techies--
Why does @TestCaseStatement in the code snippet fail to set to any value when variables are empty? I am trying to output to a table the values of variables whether they contain strings, integers or are just unset.

Also,  when I get this to work, I want to import this logic into a program that has multiple transactions -- how do I manage
the GOTO routine so as to avoid the write to the error/debug log under ProcessDEBUG: from being treated as part of an existing transaction? Finally, what happens on a return in a GOTO when called while processing a transaction?

DECLARE @dbgFlag bit
DECLARE @TestCaseStatement nvarchar(max)

SET @dbgFlag = 'True'

IF(@valTest = 'GO')
 BEGIN

-- ##DEBUG
   IF (@dbgFlg = 'True')
    BEGIN
     -- // this works because @StrikeTypeName is set to a value like 'Field'
     -- SET @TestCaseStatement = 'UPDATE s1: StrikeTypeName incoming: ' + 
     --   @StrikeTypeName + ' conditioned      lowercase: ' + @StrikeTypeName + 
     -- ' PatClassTypeID: ' + convert(varchar(10),@PatClassTypeID) 
     -- // This did NOT work, because the incoming values are not set to anything.
     -- SET @TestCaseStatement = 'UPDATE s2: ret_myDesignID incoming: ' + 
     -- convert(varchar(10),@ret_myDesignID) + ' BOPatternNum incoming: ' + 
     -- @BOPatternNum + ' BOPatternID: ' + convert(varchar(10),@BOPatternID)
     
     -- // This worked because I forced a value of '103'.
     SET @TestCaseStatement = 'UPDATE s2: ret_myDesignID incoming: ' + convert(varchar(10),103)

     PRINT @TestCaseStatement
     GOTO ProcessDEBUG

    END

 END

ProcessDEBUG:

INSERT INTo SHG_ErrorLog 
 (EntryDate,
  AppName,
  ErrorMsg
 )
VALUES
 (getdate(),
  'DASH',
  @TestCaseStatement
 )

return

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