Display warning message when attempting to package an empty model directory

[mocsharp]

When users try to package an empty model directory, the packager continues and fails with the following error:

 => CACHED [release 15/22] RUN pip install --no-cache-dir --user -r /tmp/requirements.txt                                                           0.0s
 => CACHED [release 16/22] COPY ./holoscan-2.8.0.1-cp310-cp310-manylinux_2_35_aarch64.whl /tmp/holoscan-2.8.0.1-cp310-cp310-manylinux_2_35_aarch64  0.0s
 => CACHED [release 17/22] RUN pip install /tmp/holoscan-2.8.0.1-cp310-cp310-manylinux_2_35_aarch64.whl                                             0.0s
 => ERROR [release 18/22] COPY ./models  /opt/holoscan/models                                                                                       0.0s
------
 > [release 18/22] COPY ./models  /opt/holoscan/models:
------
Dockerfile:158
--------------------
 156 |     
 157 |     
 158 | >>> COPY ./models  /opt/holoscan/models
 159 |     
 160 |     
--------------------
ERROR: failed to solve: failed to compute cache key: failed to calculate checksum of ref kfeblozzji2f87ghcuzhtwn9t::oomgezoj5urmx2l5j9e3t1erj: "/models": not found
[2024-12-16 12:43:54,278] [INFO] (packager) - Build Summary:

Platform: igx-orin-devkit/dgpu
    Status: Failure
    Error:  Error building image: see Docker output for additional details.

Instead, the packager shall display a warning and not attempt to build the container.