Deberá generar otra lista de columnas para la declaración de selección y envolver meses individuales con ISNULL, así:
DECLARE @cols AS NVARCHAR(MAX),
@selectCols AS NVARCHAR(MAX),
@query AS NVARCHAR(MAX)
SELECT @selectCols = STUFF
(
(
SELECT distinct ', ISNULL(' + QUOTENAME([MONTH]) + ', 0) AS ' + QUOTENAME([MONTH])
FROM [HEADCOUNT]
WHERE [MONTH] BETWEEN '01-012018' AND '12-01-2018'
ORDER BY ',' + QUOTENAME([MONTH])
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'),1,1,''
)
SELECT @cols = STUFF
(
(
SELECT distinct ',' + QUOTENAME([MONTH])
FROM [HEADCOUNT]
WHERE [MONTH] BETWEEN '01-012018' AND '12-01-2018'
ORDER BY ',' + QUOTENAME([MONTH])
FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'),1,1,''
)
SET @query = 'SELECT [FTE/RATE CARD],' + @selectCols + ' FROM
(
SELECT [MONTH],[FTE/RATE CARD],[HC]
FROM [HEADCOUNT]
WHERE [CC-LOC] IN ([CC-LOC])
) x pivot (Sum ([HC]) for [MONTH] in (' + @cols + '))p' execute(@query)
