fix: Remove Python 3.8 identity test warning

Description of changes:

Resolves SyntaxWarning: "is not" with a literal. Did you mean "!="? warning introduced in Python 3.8. Identity string comparison replaced with equality comparison. This warning appears in training and processing logs within SageMaker when running recent versions of Python (e.g. on recent DL AMIs/containers).

Before change:

> pytest tests/profiler/core/test_system_metric_reader.py::test_SystemLocalMetricsReader
================================================= test session starts ==================================================
platform linux -- Python 3.9.7, pytest-7.1.2, pluggy-1.0.0
rootdir: /<path>/sagemaker-debugger
collected 2 items

tests/profiler/core/test_system_metric_reader.py ..                                                              [100%]

=================================================== warnings summary ===================================================
smdebug/profiler/system_metrics_reader.py:63
  /<path>/sagemaker-debugger/smdebug/profiler/system_metrics_reader.py:63: SyntaxWarning: "is not" with a literal. Did you mean "!="?
    if self._startAfter_prefix is not "":

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
============================================= 2 passed, 1 warning in 0.21s =============================================

After change:

> pytest tests/profiler/core/test_system_metric_reader.py::test_SystemLocalMetricsReader
================================================= test session starts ==================================================
platform linux -- Python 3.9.7, pytest-7.1.2, pluggy-1.0.0
rootdir: /<path>/sagemaker-debugger
collected 2 items

tests/profiler/core/test_system_metric_reader.py ..                                                              [100%]

================================================== 2 passed in 0.29s ===================================================

Style and formatting:

I have run pre-commit install && pre-commit run --all-files to ensure that auto-formatting happens with every commit.

Issue number, if available

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awslabs / sagemaker-debugger