fix(bigtable/bttest): Error when applying a mutation with an empty ro…

…w key

Against a real version of Bigtable, an attempt to apply a mutation with
an empty row key leads to the following error being produced

```
unable to apply: rpc error: code = InvalidArgument desc = Row keys must be non-empty
```

bttest does not conform to this behaviour. This commit:

- adds error-handling when given a mutation with an empty row key, and
- adds a regression test.

bigtable/bttest/inmem.go

@@ -1072,6 +1072,13 @@ func (s *server) PingAndWarm(ctx context.Context, req *btpb.PingAndWarmRequest)
 // fam should be a snapshot of the keys of tbl.families.
 // It assumes r.mu is locked.
 func applyMutations(tbl *table, r *row, muts []*btpb.Mutation, fs map[string]*columnFamily) error {
+	if len(r.key) == 0 {
+		return status.Errorf(
+			codes.InvalidArgument,
+			"Row keys must be non-empty",
+		)
+	}
+
 	for _, mut := range muts {
 		switch mut := mut.Mutation.(type) {
 		default:

bigtable/bttest/inmem_test.go

@@ -2424,6 +2424,48 @@ func TestMutateRowsEmptyMutationErrors(t *testing.T) {
 	}
 }
 
+func TestMutateRowEmptyRowKeyErrors(t *testing.T) {
+	srv := &server{tables: make(map[string]*table)}
+	ctx := context.Background()
+
+	const tableID = "mytable"
+	tblReq := &btapb.CreateTableRequest{
+		Parent:  "cluster",
+		TableId: tableID,
+		Table: &btapb.Table{
+			ColumnFamilies: map[string]*btapb.ColumnFamily{"cf": {}},
+		},
+	}
+	if _, err := srv.CreateTable(ctx, tblReq); err != nil {
+		t.Fatalf("Failed to create the table: %v", err)
+	}
+
+	const name = "cluster/tables/" + tableID
+	req := &btpb.MutateRowRequest{
+		TableName: name,
+		RowKey:    []byte(""),
+		Mutations: []*btpb.Mutation{
+			{
+				Mutation: &btpb.Mutation_SetCell_{
+					SetCell: &btpb.Mutation_SetCell{
+						FamilyName:      "cf",
+						ColumnQualifier: []byte("col"),
+						TimestampMicros: 1000,
+						Value:           []byte("hello, world!"),
+					},
+				},
+			},
+		},
+	}
+
+	resp, err := srv.MutateRow(ctx, req)
+	if resp != nil || err == nil || err.Error() !=
+		"rpc error: code = InvalidArgument"+
+			" desc = Row keys must be non-empty" {
+		t.Fatalf("Failed to produce the expected error: %s", err)
+	}
+}
+
 func TestFilterRowCellsPerRowLimitFilterTruthiness(t *testing.T) {
 	row := &row{
 		key: "row",