fix SyntaxWarning for comparing with a literal

[aksakalli]

Summary

This fixes the following warning we get for chart_studio:

/opt/conda/lib/python3.8/site-packages/chart_studio/tools.py:290: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if share_key is "":

We should be comparing the values for literals. Alternatively we can write this comparison as

    if not share_key:
        return "{plotly_rest_url}/~{file_owner}/{file_id}.embed".format(
            plotly_rest_url=plotly_rest_url, file_owner=file_owner, file_id=file_id
        )

Documentation PR

• [x] I've seen the doc/README.md file
• [x] This change runs in the current version of Plotly on PyPI and targets the doc-prod branch OR it targets the master branch
• [ ] If this PR modifies the first example in a page or adds a new one, it is a px example if at all possible
• [ ] Every new/modified example has a descriptive title and motivating sentence or paragraph
• [ ] Every new/modified example is independently runnable
• [ ] Every new/modified example is optimized for short line count and focuses on the Plotly/visualization-related aspects of the example rather than the computation required to produce the data being visualized
• [ ] Meaningful/relatable datasets are used for all new examples instead of randomly-generated data where possible
• [ ] The random seed is set if using randomly-generated data in new/modified examples
• [ ] New/modified remote datasets are loaded from https://plotly.github.io/datasets and added to https://github.com/plotly/datasets
• [ ] Large computations are avoided in the new/modified examples in favour of loading remote datasets that represent the output of such computations
• [ ] Imports are plotly.graph_objects as go / plotly.express as px / plotly.io as pio
• [ ] Data frames are always called df
• [ ] fig = <something> call is high up in each new/modified example (either px.<something> or make_subplots or go.Figure)
• [ ] Liberal use is made of fig.add_* and fig.update_* rather than go.Figure(data=..., layout=...) in every new/modified example
• [ ] Specific adders and updaters like fig.add_shape and fig.update_xaxes are used instead of big fig.update_layout calls in every new/modified example
• [ ] fig.show() is at the end of each new/modified example
• [ ] plotly.plot() and plotly.iplot() are not used in any new/modified example
• [ ] Hex codes for colors are not used in any new/modified example in favour of these nice ones

Code PR

• [x] I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• [ ] I have added tests (if submitting a new feature or correcting a bug) or modified existing tests.
• [ ] For a new feature, I have added documentation examples in an existing or new tutorial notebook (please see the doc checklist as well).
• [ ] I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• [ ] For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).