Keep the existing default catalog for SessionStateBuilder::new_from_existing

[goldmedal]

Which issue does this PR close?

Closes #11988 .

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[alamb]

Thanks @goldmedal -- this makes sense to me

[alamb]

I ran this test locally without the code changes in this PR and the test fails (as expected) thus verifying test coverage for the new code 👍

assertion failed: new_state.catalog_list().catalog(default_catalog.as_str()).unwrap().schema(default_schema.as_str()).unwrap().table_exist("employee")
thread 'execution::session_state::tests::test_from_existing' panicked at datafusion/core/src/execution/session_state.rs:1887:9:
assertion failed: new_state.catalog_list().catalog(default_catalog.as_str()).unwrap().schema(default_schema.as_str()).unwrap().table_exist("employee")
stack backtrace:
   0: rust_begin_unwind
             at /rustc/3f5fd8dd41153bc5fdca9427e9e05be2c767ba23/library/std/src/panicking.rs:652:5
   1: core::panicking::panic_fmt
             at /rustc/3f5fd8dd41153bc5fdca9427e9e05be2c767ba23/library/core/src/panicking.rs:72:14
   2: core::panicking::panic
             at /rustc/3f5fd8dd41153bc5fdca9427e9e05be2c767ba23/library/core/src/panicking.rs:146:5
   3: datafusion::execution::session_state::tests::test_from_existing
             at ./src/execution/session_state.rs:1887:9
   4: datafusion::execution::session_state::tests::test_from_existing::{{closure}}
             at ./src/execution/session_state.rs:1848:32
   5: core::ops::function::FnOnce::call_once
             at /rustc/3f5fd8dd41153bc5fdca9427e9e05be2c767ba23/library/core/src/ops/function.rs:250:5
   6: core::ops::function::FnOnce::call_once
             at /rustc/3f5fd8dd41153bc5fdca9427e9e05be2c767ba23/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[alamb]

Thanks again @goldmedal

[goldmedal]

Thanks @alamb ❤️