fix(bigtable): Allow nil condition in conditional mutation

[bhshkh]

When the test proxy receives a conditional mutation request with nil condition, it uses the PassAllFilter to call Apply method of Go library. Thus, the request sent by Go library to Cloud Bigtable service contains the field pass all filter.
TestCheckAndMutateRow_NoRetry_TrueMutations conformance test fails with error

=== RUN   TestCheckAndMutateRow_NoRetry_TrueMutations
    checkandmutaterow_test.go:138: diff found (-want +got):
          (*bigtablepb.CheckAndMutateRowRequest)(Inverse(protocmp.Transform, protocmp.Message{
          	"@type":            s"google.bigtable.v2.CheckAndMutateRowRequest",
        + 	"predicate_filter": s"{pass_all_filter:true}",
          	"row_key":          []uint8("row-01"),
          	"table_name":       string("projects/project/instances/instance/tables/table"),
          	"true_mutations":   []protocmp.Message{{"@type": s"google.bigtable.v2.Mutation", ...}, {"@type": s"google.bigtable.v2.Mutation", ...}},
          }))
--- FAIL: TestCheckAndMutateRow_NoRetry_TrueMutations (0.00s)

The Go library assumes that the field cond in Mutation being non-nil denotes conditional mutation but as per the above test we want to allow conditional mutation with no condition. So, a new variable isConditional is being added to Mutation to distinguish between conditional and non-conditional mutations.

After fix:

=== RUN   TestCheckAndMutateRow_NoRetry_TrueMutations
--- PASS: TestCheckAndMutateRow_NoRetry_TrueMutations (0.00s)

[gkevinzheng]

Where is m.isConditional ever set to false?

[bhshkh]

Where is m.isConditional ever set to false?

If it is not set, that means it is false. Default value of bool types is false