SQL Server 2019

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CREATE TABLE T(MyJSONColumn nvarchar(max) CHECK (ISJSON([MyJSONColumn]) = (1)))
INSERT T VALUES(NULL)
1 rows affected

INSERT T VALUES('')
Msg 547 Level 16 State 0 Line 1
The INSERT statement conflicted with the CHECK constraint "CK__T__MyJSONColumn__239E4DCF". The conflict occurred in database "fiddle_42909cf8861644c190072e174a19045c", table "dbo.T", column 'MyJSONColumn'.
Msg 3621 Level 0 State 0 Line 1
The statement has been terminated.

INSERT T VALUES('{}')
1 rows affected