Tracing with tracing_group_mode=Browser fails with IndexError when a context is created from plain Python #5357

Describe the bug
With tracing_group_mode=Browser, creating a traced context outside a Robot Framework keyword fails with IndexError: list index out of range. Browser names the context's trace group after the last keyword call, and from plain Python there is none.

To Reproduce

from Browser import Browser
from Browser.utils.data_types import TracingGroupMode

browser = Browser(tracing_group_mode=TracingGroupMode.Browser)
browser.new_browser()
browser.new_context(tracing=True)   # IndexError

New Persistent Context and register_adopted_context (new in #5326) fail the same way.

Expected behavior
The context is traced. It gets no trace groups, because trace groups exist only for keyword calls, as with tracing_group_mode=Full, which already handles this case.

Fix
#5356. Affects 20.6.0 and earlier.