Fix NullReferenceException in SqlServerAnnotationCodeGenerator when scaffolding columns with named default constraints and CLR default values

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Description

Fixes #37175

Scaffolding fails with NullReferenceException when a column has both a named default constraint and a default value equal to the CLR default (e.g., DEFAULT ((0)) for int).

Root cause: SqlServerAnnotationCodeGenerator.GenerateFluentApiCalls assumed that if DefaultConstraintName exists, either DefaultValue or DefaultValueSql must also exist. However, the base scaffolding logic in ScaffoldingModelExtensions.GenerateAnnotations can remove these annotations when the default value equals the CLR default, leaving only DefaultConstraintName.

Fix: Changed the Check.DebugAssert to a conditional else if check. When neither DefaultValue nor DefaultValueSql exists, skip generating code for the constraint name instead of throwing.

-- This table definition triggers the bug
CREATE TABLE TestTable (
    Id INT NOT NULL PRIMARY KEY,
    Value INT NOT NULL CONSTRAINT DF_TestTable_Value DEFAULT ((0))
);

Original prompt

Fix #37175. Start by creating a test that fails with the same or similar exception

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