How Do I Check , If Column Value Is Not Null, Then Use Equal Sign, Else Ignore The Filter

Create table A
(
	KPI_ID Int,
	Unit decimal, 
	Frequency decimal,
)


Create table B
(
	KPI_ID Int,
	Value decimal, 
	ValueDate Datetime,
)
-- Please insert the value you want
-- Now run the query as below
select * from A ta left join B tb on ta.KPI_ID = tb.KPI_ID

Create table table1
(
	KPI_ID Int,
	Unit decimal, 
	Frequency decimal,
)


Create table table2
(
	KPI_ID Int,
	Value decimal, 
	ValueDate Datetime,
)

select * from table1
Go
select * from table1
Go

Insert into table1 (KPI_ID, Unit, Frequency)values(1001, 10, 15)
Insert into table1 (KPI_ID, Unit, Frequency)values(1002, 10, 15)
Insert into table1 (KPI_ID, Unit, Frequency)values(1003, 10, 15)
Insert into table1 (KPI_ID, Unit, Frequency)values(1004, 10, 15)
Insert into table1 (KPI_ID, Unit, Frequency)values(1005, 10, 15)
Insert into table1 (KPI_ID, Unit, Frequency)values(1006, 10, 15)
Insert into table1 (KPI_ID, Unit, Frequency)values(1007, 10, 15)
GO
Insert into table2 (KPI_ID, Value, ValueDate)values(1001, 55, '20140131')
Insert into table2 (KPI_ID, Value, ValueDate)values(1002, 50, '20140131')
Insert into table2 (KPI_ID, Value, ValueDate)values(1002, 54, '20140228')
Insert into table2 (KPI_ID, Value, ValueDate)values(1003, 53, '20140131')
Insert into table2 (KPI_ID, Value, ValueDate)values(1003, 57, '20140228')
GO

--1, 1001, 10, 15, 55, 20140131
--2, 1002, 10, 15, 50, 20140131
--3, 1002, 10, 15, 54, 20140231
--4, 1003, 10, 15, 53, 20140131
--5, 1003, 10, 15, 57, 20140228
--6, 1004, 10, 15, null, null 
--7, 1005, 10, 15, null, null 
--8, 1006, 10, 15, null , null

-- Please insert the value you want
-- Now run the query as below
select * from table1 ta left join table2 tb on ta.KPI_ID = tb.KPI_ID WHERE CONVERT(VARCHAR(24),tb.ValueDate,112) like '201401%' OR CONVERT(VARCHAR(24),tb.ValueDate,112) IS NULL