stdscript: Add v0 nulldata benchmark.

BenchmarkIsNullDataScript
-------------------------
v0_complex_non_standard          279362058   4.306 ns/op   0 allocs/op
v0_nulldata_no_data_push         276928082   4.325 ns/op   0 allocs/op
v0_nulldata_single_zero_push     40513850    29.47 ns/op   0 allocs/op
v0_nulldata_small_int_push       39842752    29.65 ns/op   0 allocs/op
v0_nulldata_max_small_int_push   36889138    29.82 ns/op   0 allocs/op
v0_nulldata_small_data_push      40597321    28.86 ns/op   0 allocs/op
v0_nulldata_60-byte_push         41307235    28.68 ns/op   0 allocs/op
v0_nulldata_max_standard_push    36347774    32.41 ns/op   0 allocs/op

This is part of a series of commits to fully implement the stdscript
package.

internal/staging/stdscript/script_bench_test.go

@@ -184,3 +184,12 @@ func BenchmarkMultiSigRedeemScriptFromScriptSigV0(b *testing.B) {
 		})
 	}
 }
+
+// BenchmarkIsNullDataScript benchmarks the performance of analyzing various
+// public key scripts to determine if they are nulldata scripts.
+func BenchmarkIsNullDataScript(b *testing.B) {
+	filterFn := func(test scriptTest) bool {
+		return test.wantType == STNullData
+	}
+	benchIsX(b, filterFn, IsNullDataScript)
+}