db: fix Options.Parse handling of unrecognized comparer

In b3993ff the behavior of Options.Parse when no NewComparer hook is provided
changed. Parse began to nil out the Comparer field, clearing any existing
Comparer.

options.go

@@ -1667,7 +1667,11 @@ func (o *Options) Parse(s string, hooks *ParseHooks) error {
 					}
 				}
 			case "comparer":
-				o.Comparer, err = parseComparer(value)
+				var comparer *Comparer
+				comparer, err = parseComparer(value)
+				if comparer != nil {
+					o.Comparer = comparer
+				}
 			case "compaction_debt_concurrency":
 				o.Experimental.CompactionDebtConcurrency, err = strconv.ParseUint(value, 10, 64)
 			case "delete_range_flush_delay":

options_test.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/cockroachdb/errors"
 	"github.com/cockroachdb/pebble/internal/base"
+	"github.com/cockroachdb/pebble/internal/testkeys"
 	"github.com/cockroachdb/pebble/vfs"
 	"github.com/cockroachdb/pebble/wal"
 	"github.com/stretchr/testify/require"
@@ -317,6 +318,16 @@ func TestOptionsParse(t *testing.T) {
 	}
 }
 
+func TestOptionsParseComparerOverwrite(t *testing.T) {
+	// Test that an unrecognized comparer in the OPTIONS file does not nil out
+	// the Comparer field.
+	o := &Options{Comparer: testkeys.Comparer}
+	err := o.Parse(`[Options]
+comparer=unrecognized`, nil)
+	require.NoError(t, err)
+	require.Equal(t, testkeys.Comparer, o.Comparer)
+}
+
 func TestOptionsValidate(t *testing.T) {
 	testCases := []struct {
 		options  string