[telemetry] Fix benchmark smoke unittest _disabled_strings logic, take 2.

tools/perf/benchmarks/benchmark_smoke_unittest.py

Index: tools/perf/benchmarks/benchmark_smoke_unittest.py
diff --git a/tools/perf/benchmarks/benchmark_smoke_unittest.py b/tools/perf/benchmarks/benchmark_smoke_unittest.py
index 97dd867e1e715ff879590f32b09607a6fdce3eb2..ea0b9d401841e4ec0db9c61f2c7cd01a6fb26237 100644
--- a/tools/perf/benchmarks/benchmark_smoke_unittest.py
+++ b/tools/perf/benchmarks/benchmark_smoke_unittest.py
@@ -106,14 +106,16 @@ def load_tests(_, _2, _3):
     # test from the class. We should probably discover all of the tests
     # in a class, and then throw the ones we don't need away instead.
     if hasattr(benchmark, '_enabled_strings'):
-      if hasattr(method, '_enabled_strings'):
+      if (hasattr(method, '_enabled_strings') and
+          method._enabled_strings and benchmark._enabled_strings):
         method._enabled_strings += benchmark._enabled_strings
-      else:
+      elif not hasattr(method, '_enabled_strings') or method._enabled_strings:
         method._enabled_strings = benchmark._enabled_strings
     if hasattr(benchmark, '_disabled_strings'):
-      if hasattr(method, '_disabled_strings'):
+      if (hasattr(method, '_disabled_strings') and
+          method._disabled_strings and benchmark._disabled_strings):
         method._disabled_strings += benchmark._disabled_strings
-      else:
+      elif not hasattr(method, '_disabled_strings') or method._disabled_strings:
         method._disabled_strings = benchmark._disabled_strings
     setattr(BenchmarkSmokeTest, benchmark.Name(), method)