feat: support empty URL for local backends in ibis.connect()

[lostmygithubaccount]

Is your feature request related to a problem?

often when testing against multiple backends, myself or a user will do something like:

backend = "polars"

if backend == "polars":
    con = ibis.polars.connect()
...

Ibis supports a cleaner ibis.connect() that takes a backend-specific URI. those don't make sense for many of the local backends and are currently NotImplemented, but it would be nice for consistency and ease of use to support empty URIs for all of:

• pandas
• polars
• dask
• datafusion

and any others not supported now. should also support #8435 and #7710 once those are in (and any other future local backends)

Describe the solution you'd like

backend = "polars" # "duckdb" "pandas" "datafusion" "dask"

import ibis

con = ibis.connect(f"{backend}://")

adding anything to the URIs could warn or error

What version of ibis are you running?

main

What backend(s) are you using, if any?

duckdb/pandas/datafusion/polars/dask/pyspark

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