Avoid useless deepcopy of target with custom pulse gates in transpile (…

…#10973)

* Avoid useless deepcopy of target in transpile

* Fix lint

qiskit/compiler/transpiler.py

@@ -300,6 +300,7 @@ def callback_func(**kwargs):
         )
 
     _skip_target = False
+    _given_inst_map = bool(inst_map)  # check before inst_map is overwritten
     # If a target is specified have it override any implicit selections from a backend
     if target is not None:
         if coupling_map is None:
@@ -335,7 +336,7 @@ def callback_func(**kwargs):
 
     timing_constraints = _parse_timing_constraints(backend, timing_constraints)
 
-    if inst_map is not None and inst_map.has_custom_gate() and target is not None:
+    if _given_inst_map and inst_map.has_custom_gate() and target is not None:
         # Do not mutate backend target
         target = copy.deepcopy(target)
         target.update_from_instruction_schedule_map(inst_map)