Two changes to command-line scripts (#4235)

During install testing I discovered two small problems in the
command-line scripts. These are fixed.

## What type of PR is this? (check all applicable)

- [X Bug Fix

## Have you discussed this change with the InvokeAI team?
- [X] Yes
- 
      
## Have you updated all relevant documentation?
- [X] Yes


## Description

- installer - use correct entry point for invokeai-configure
- model merge script - prevent error when `--root` not provided

installer/lib/installer.py

@@ -348,7 +348,7 @@ class InvokeAiInstance:
 
         introduction()
 
-        from invokeai.frontend.install import invokeai_configure
+        from invokeai.frontend.install.invokeai_configure import invokeai_configure
 
         # NOTE: currently the config script does its own arg parsing! this means the command-line switches
         # from the installer will also automatically propagate down to the config script.

invokeai/frontend/merge/merge_diffusers.py

@@ -382,7 +382,8 @@ def run_cli(args: Namespace):
 
 def main():
     args = _parse_args()
-    config.parse_args(["--root", str(args.root_dir)])
+    if args.root_dir:
+        config.parse_args(["--root", str(args.root_dir)])
 
     try:
         if args.front_end: