Avoid importing dask-expr if "query-planning" config is False (#15340)

During some offline debugging with @bdice and @divyegala, we discovered that some cuml tests are somehow failing after `dask_expr` is imported - Even if `dask_expr` is not actually being used. I'd like to figure out exactly what is causing that problem, but the first thing we can/should do is avoid the import altogether when the "query-planning" config is set to `False`.

Authors:
  - Richard (Rick) Zamora (https://github.com/rjzamora)

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - GALI PREM SAGAR (https://github.com/galipremsagar)

URL: #15340

python/dask_cudf/dask_cudf/expr/__init__.py

@@ -7,12 +7,12 @@
 QUERY_PLANNING_ON = config.get("dataframe.query-planning", None) is not False
 
 # Register custom expressions and collections
-try:
-    import dask_cudf.expr._collection
-    import dask_cudf.expr._expr
+if QUERY_PLANNING_ON:
+    try:
+        import dask_cudf.expr._collection
+        import dask_cudf.expr._expr
 
-except ImportError as err:
-    if QUERY_PLANNING_ON:
+    except ImportError as err:
         # Dask *should* raise an error before this.
         # However, we can still raise here to be certain.
         raise RuntimeError(