No ‘null’ output if command doesn’t return (#1913)

- This prevents commands from printing ‘null’ in JSON output
- But still keeps ‘[]’, ‘{}’, and ‘false’ as valid JSON results

Closes #1908

Author: derekbekoe

src/azure-cli-core/azure/cli/core/test_utils/vcr_test_base.py

@@ -164,7 +164,7 @@ def __init__(self):
         pass
 
     def compare(self, data):  # pylint: disable=no-self-use
-        none_strings = ['[]', '{}', 'null', 'false']
+        none_strings = ['[]', '{}', 'false']
         try:
             assert not data or data in none_strings
         except AssertionError:

src/azure-cli/azure/cli/main.py

@@ -38,7 +38,7 @@ def main(args, file=sys.stdout):  # pylint: disable=redefined-builtin
 
         # Commands can return a dictionary/list of results
         # If they do, we print the results.
-        if cmd_result:
+        if cmd_result and cmd_result.result is not None:
             from azure.cli.core._output import OutputProducer
             formatter = OutputProducer.get_formatter(APPLICATION.configuration.output_format)
             OutputProducer(formatter=formatter, file=file).out(cmd_result)