webagg: Don't resize canvas if WebSocket isn't connected

[QuLogic]

PR summary

When opening embedding_webagg_sgskip.py in Firefox, the empty canvas slowly increases in height until the figure first comes through. Skipping the canvas resize until the WebSocket connects prevents this unnecessary canvas change.

• If it is still connecting, then we will get an initial resize from Python once it connects.
• If it has disconnected, then resizing will clear the canvas and never get anything back to refill it, so better to not resize and keep something visible.

PR checklist

• [n/a] "closes #0000" is in the body of the PR description to link the related issue
• [n/a] new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• [n/a] New Features and API Changes are noted with a directive and release note
• [n/a] Documentation complies with general and docstring guidelines