Concatenating NULL returns a value in T-SQL (CONCAT function)

Question

Why does this return . when col1 contains a blank value?

CONCAT(NULLIF([COL1],''),'.')

I have 3 columns that i need to concatenate with a . in between, sometimes the column contains a blank value. In that case the trailing . should not be concatenated. What functions do I use?

col1 col2 col3 
A     1    x
B     2    

expected results:

A.1.X
B.2

test code:

DECLARE @tbl TABLE(a varchar(100),b varchar(100),c varchar(100))
INSERT INTO @tbl
SELECT 'A','1','X' UNION
SELECT 'B','2','' UNION
SELECT 'C','','' UNION
SELECT '','1','X' UNION
SELECT 'B','','' UNION
SELECT 'C','',''

SELECT CONCAT ( Nullif(a,''),'.' + nullif(b,''), '.' + nullif(c,'')) AS Contact_Result FROM @tbl;

Answer 1

THIS IS AN ANSWER THAT COVERS ALL POSSIBILITIES

SELECT SUBSTRING(CONCAT ('.' + NULLIF(a,''),'.' + NULLIF(b,''),'.' + NULLIF(c,'')),2,10000) AS Contact_Result FROM @tbl;  

Complete test cases

DECLARE @tbl TABLE(a varchar(100),b varchar(100),c varchar(100))
INSERT INTO @tbl
SELECT 'a','','' UNION
SELECT 'a','b','' UNION
SELECT 'a','b','c' UNION
SELECT 'a','','c' UNION
SELECT '','b','c' UNION
SELECT '','b','' UNION
SELECT '','','c' UNION
SELECT '','','' 


SELECT SUBSTRING(CONCAT ('.' + NULLIF(a,''),'.' + NULLIF(b,''),'.' + NULLIF(c,'')),2,10000) AS Contact_Result FROM @tbl;  

Results

c
b
b.c
a
a.c
a.b
a.b.c