feat: add MutatePresentColumns method

Context:
Spanner has a hard limit on size of a mutation that is 20k cells, the
product of rows and columns. Schemas for protobuf messages containing
oneofs could be mapped to large number of nullable fields in Spanner.

When inserting rows to this schema, the majority of these nullable
fields will not be set, and setting the column to null is equivalent to
not setting it at all. This method, not setting columns at all,
drastically lowers the number of cells mutated in a mutation.

---

This change adds `row.MutatePresentColumns` which only mutates nullable
fields that are present.

* Nullable scalars are considered present if `column.Valid == true`
* Nullable arrays are considered present if `len(column) != 0`
* Nullable bytes are considered present if `len(colum) != 0`

internal/codegen/databasecodegen/row.go

@@ -67,6 +67,7 @@ func (g RowCodeGenerator) GenerateCode(f *codegen.File) {
 	g.generateUnmarshalFunction(f)
 	g.generateMutationFunction(f)
 	g.generateMutationForColumnsFunction(f)
+	g.generateMutationForPresentColumnsFunction(f)
 	g.generatePrimaryKeyMethod(f)
 }
 
@@ -171,6 +172,46 @@ func (g RowCodeGenerator) generateMutationForColumnsFunction(f *codegen.File) {
 	f.P("}")
 }
 
+func (g RowCodeGenerator) generateMutationForPresentColumnsFunction(f *codegen.File) {
+	f.P()
+	f.P("func (r *", g.Type(), ") MutatePresentColumns() (string, []string, []interface{}) {")
+	f.P("columns := make([]string, 0, len(r.", g.ColumnNamesMethod(), "()))")
+	// non-nullable fields
+	f.P("columns = append(")
+	f.P("columns,")
+	for _, column := range g.Table.Columns {
+		if column.NotNull {
+			f.P(strconv.Quote(string(column.Name)), ",")
+		}
+	}
+	f.P(")")
+	// nullable fields
+	for _, column := range g.Table.Columns {
+		if column.NotNull {
+			continue
+		}
+		f.P("if ", g.isPresentPredicate(column), " {")
+		f.P("columns = append(columns, ", strconv.Quote(string(column.Name)), ")")
+		f.P("}")
+	}
+	f.P("return r.MutateColumns(columns)")
+	f.P("}")
+}
+
+func (g RowCodeGenerator) isPresentPredicate(column *spanddl.Column) string {
+	switch {
+	case column.Type.Array:
+		return "len(r." + g.ColumnFieldName(column) + ") != 0"
+	case column.Type.Base == spansql.Bytes:
+		return "len(r." + g.ColumnFieldName(column) + ") != 0"
+	case !column.NotNull:
+		return "!r." + g.ColumnFieldName(column) + ".IsNull()"
+	default:
+		// columns that are non-nullable are always considered present
+		return "true"
+	}
+}
+
 func (g RowCodeGenerator) generateColumnNamesFunctions(f *codegen.File) {
 	spansqlPkg := f.Import("cloud.google.com/go/spanner/spansql")
 	f.P()