Fix NULL default bug when merging branches with different schemas

go/libraries/doltcore/sqle/enginetest/dolt_queries_schema_merge.go

@@ -644,6 +644,41 @@ var SchemaChangeTestsBasicCases = []MergeScriptTest{
 			},
 		},
 	},
+	{
+		// One branch adds a new column with NULL as default
+		// Other branch has new rows which need to be migrated.
+		// Created values are NULL, not "NULL".
+		Name: "creating new column to replace ancestor column",
+		AncSetUpScript: []string{
+			"CREATE table t (pk int primary key);",
+			"INSERT into t values (1), (2);",
+		},
+		RightSetUpScript: []string{
+			"ALTER TABLE t ADD new_col LONGTEXT DEFAULT NULL",
+			"INSERT INTO t VALUES (3, 'three'), (4, 'four');",
+		},
+		LeftSetUpScript: []string{
+			// Put rows on main which are not in the right branch.
+			"INSERT INTO t VALUES (5), (6)",
+		},
+		Assertions: []queries.ScriptTestAssertion{
+			{
+				Query:    "call dolt_merge('right');",
+				Expected: []sql.Row{{doltCommit, 0, 0, "merge successful"}},
+			},
+			{
+				Query: "select * from t;",
+				Expected: []sql.Row{
+					{1, nil},
+					{2, nil},
+					{3, "three"},
+					{4, "four"},
+					{5, nil},
+					{6, nil},
+				},
+			},
+		},
+	},
 }
 
 var SchemaChangeTestsCollations = []MergeScriptTest{

go/libraries/doltcore/sqle/sqlfmt/schema_fmt.go

@@ -193,7 +193,7 @@ func GenerateCreateTableIndentedColumnDefinition(col schema.Column, tableCollati
 	var defaultVal, genVal, onUpdateVal *sql.ColumnDefaultValue
 	if col.Default != "" {
 		// hacky way to determine if column default is an expression
-		if col.Default[0] != '(' && col.Default[len(col.Default)-1] != ')' && col.Default[0] != '\'' && col.Default[len(col.Default)-1] != '\'' {
+		if col.Default[0] != '(' && col.Default[len(col.Default)-1] != ')' && col.Default[0] != '\'' && col.Default[len(col.Default)-1] != '\'' && col.Default != "NULL" {
 			col.Default = fmt.Sprintf("'%s'", col.Default)
 		}
 		defaultVal = sql.NewUnresolvedColumnDefaultValue(col.Default)