[tests] Fix temporary environment variable setting.

compiler_gym/envs/llvm/llvm_benchmark.py

@@ -119,7 +119,7 @@ def get_system_library_flags(compiler: Optional[str] = None) -> List[str]:
     :raises OSError: If the compiler fails, or if the output of the compiler
         cannot be understood.
     """
-    compiler = compiler or os.environ.get("CXX", "c++")
+    compiler = compiler or (os.environ.get("CXX") or "c++")
     # We want to cache the results of this expensive query, but also emit a
     # logging warning when the function is called, including when the call
     # results in a cache hit. We therefore must cache both the flags and the

tests/llvm/llvm_benchmark_test.py

@@ -59,15 +59,14 @@ def test_get_system_library_flags_nonzero_exit_status(caplog):
 
 def test_get_system_library_flags_output_parse_failure(caplog):
     """Test that setting the $CXX to an invalid binary raises an error."""
-    old_cxx = os.environ.get("CXX")
+    old_cxx = os.environ.get("CXX", "")
     try:
         os.environ["CXX"] = "echo"
         flags, error = llvm_benchmark._get_cached_system_library_flags("echo")
         assert flags == []
         assert "Failed to parse '#include <...>' search paths from echo" in error
     finally:
-        if old_cxx:
-            os.environ["CXX"] = old_cxx
+        os.environ["CXX"] = old_cxx
 
 
 def test_get_system_library_flags():