Check for None before lazy loading

[thomasjpfan]

Tracking issue

Related to flyteorg/flyte#4877

Why are the changes needed?

From time to time, I get this error locally or remotely:

Traceback (most recent call last):
  File "/opt/conda/envs/envd/bin/pyflyte-execute", line 5, in <module>
    from flytekit.bin.entrypoint import execute_task_cmd
  File "/opt/conda/envs/envd/lib/python3.11/site-packages/flytekit/__init__.py", line 242, in <module>
    from flytekit.deck import Deck
  File "/opt/conda/envs/envd/lib/python3.11/site-packages/flytekit/deck/__init__.py", line 22, in <module>
    from .renderer import MarkdownRenderer, SourceCodeRenderer, TopFrameRenderer
  File "/opt/conda/envs/envd/lib/python3.11/site-packages/flytekit/deck/renderer.py", line 13, in <module>
    pandas = lazy_module("pandas")
             ^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/envs/envd/lib/python3.11/site-packages/flytekit/lazy_import/lazy_module.py", line 41, in lazy_module
    loader = importlib.util.LazyLoader(spec.loader)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib.util>", line 302, in __init__
  File "<frozen importlib.util>", line 293, in __check_eager_loader
TypeError: loader must define exec_module()

What changes were proposed in this pull request?

In some environments, importlib.util.find_spec returns a spec that does not have a loader.

[eapolinario]

Very interesting that this is non-deterministic (for a given package version). I wonder if it's worth logging that case also.