Ignore the FLATC_EXECUTABLE env var when empty

[dbort]

We've seen some situations fail because flatc_path is an empty string. This seems like a likely culprit, but doesn't hurt either way.

Test Plan:
Ran the following script with (. /tmp/run.sh), which successfully created /tmp/et-test/executorch/add.pte.

/tmp/run.sh:

set -euxo pipefail

conda remove -yn et-test --all
conda create -yn et-test python=3.10
conda activate et-test

mkdir -p /tmp/et-test
cd /tmp/et-test
# Get this PR
git clone -b flatc-empty-path https://github.com/dbort/executorch-1
cd executorch

git submodule sync
git submodule update --init

./install_requirements.sh

cat > /tmp/export_add.py << HERE
import torch
from torch.export import export
from executorch.exir import to_edge

# Start with a PyTorch model that adds two input tensors (matrices)
class Add(torch.nn.Module):
  def __init__(self):
    super(Add, self).__init__()

  def forward(self, x: torch.Tensor, y: torch.Tensor):
      return x + y

# 1. torch.export: Defines the program with the ATen operator set.
aten_dialect = export(Add(), (torch.ones(1), torch.ones(1)))

# 2. to_edge: Make optimizations for Edge devices
edge_program = to_edge(aten_dialect)

# 3. to_executorch: Convert the graph to an ExecuTorch program
executorch_program = edge_program.to_executorch()

# 4. Save the compiled .pte program
with open("add.pte", "wb") as file:
    file.write(executorch_program.buffer)
HERE

python /tmp/export_add.py
echo "Created" *.pte

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/3612

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✅ No Failures

As of commit 1ed61d4 with merge base 0e7955d ():
💚 Looks good so far! There are no failures yet. 💚

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[facebook-github-bot]

@dbort has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@dbort merged this pull request in 2b1ed26.