sql: avoid invalid SQL in raw_config_sql in crdb_internal.zones

Previously, we could include an invalid `ALTER ... CONFIGURE ZONE
USING;` stmt in the `raw_config_sql` column of `crdb_internal.zones`.
This was the case when some of the config was inherited and we didn't
want to include it for the corresponding object. The contract of this
virtual tables allows NULLs in this column, so this commit returns NULL
in such cases. Note that I spent non-trivial amount of time trying to
figure out how we could end up in this situation but was unsuccessful.

This invalid stmt was included into the output of SHOW CREATE TABLE
which, among other things, we use when recreating the bundle, so we
should now have easier time recreating multi-region bundles.

Release note: None

pkg/sql/show_zone_config.go

@@ -158,34 +158,35 @@ func getShowZoneConfigRow(
 }
 
 // zoneConfigToSQL pretty prints a zone configuration as a SQL string.
-func zoneConfigToSQL(zs *tree.ZoneSpecifier, zone *zonepb.ZoneConfig) (string, error) {
+func zoneConfigToSQL(zs *tree.ZoneSpecifier, zone *zonepb.ZoneConfig) (tree.Datum, error) {
 	constraints, err := yamlMarshalFlow(zonepb.ConstraintsList{
 		Constraints: zone.Constraints,
 		Inherited:   zone.InheritedConstraints})
 	if err != nil {
-		return "", err
+		return tree.DNull, err
 	}
 	constraints = strings.TrimSpace(constraints)
 	voterConstraints, err := yamlMarshalFlow(zonepb.ConstraintsList{
 		Constraints: zone.VoterConstraints,
 		Inherited:   zone.InheritedVoterConstraints(),
 	})
 	if err != nil {
-		return "", err
+		return tree.DNull, err
 	}
 	voterConstraints = strings.TrimSpace(voterConstraints)
 	prefs, err := yamlMarshalFlow(zone.LeasePreferences)
 	if err != nil {
-		return "", err
+		return tree.DNull, err
 	}
 	prefs = strings.TrimSpace(prefs)
 
-	useComma := false
+	first := true
 	maybeWriteComma := func(f *tree.FmtCtx) {
-		if useComma {
+		if !first {
 			f.Printf(",\n")
+		} else {
+			first = false
 		}
-		useComma = true
 	}
 
 	f := tree.NewFmtCtx(tree.FmtParsable)
@@ -228,7 +229,13 @@ func zoneConfigToSQL(zs *tree.ZoneSpecifier, zone *zonepb.ZoneConfig) (string, e
 		maybeWriteComma(f)
 		f.Printf("\tlease_preferences = %s", lexbase.EscapeSQLString(prefs))
 	}
-	return f.String(), nil
+	if first {
+		// We didn't include any zone config parameters, so rather than
+		// returning an invalid 'ALTER ... CONFIGURE ZONE USING;' stmt we'll
+		// return NULL.
+		return tree.DNull, nil
+	}
+	return tree.NewDString(f.String()), nil
 }
 
 // generateZoneConfigIntrospectionValues creates a result row
@@ -293,11 +300,12 @@ func generateZoneConfigIntrospectionValues(
 	if zs == nil {
 		values[rawConfigSQLCol] = tree.DNull
 	} else {
-		sqlStr, err := zoneConfigToSQL(zs, zone)
+		var d tree.Datum
+		d, err = zoneConfigToSQL(zs, zone)
 		if err != nil {
 			return err
 		}
-		values[rawConfigSQLCol] = tree.NewDString(sqlStr)
+		values[rawConfigSQLCol] = d
 	}
 
 	// Populate the protobuf column.