Safely build PyKE rule base.

Travis-CI testing has failed several times with the following error:

```
======================================================================
ERROR: system_test_supported_filetypes (iris.tests.system_test.SystemInitialTest)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/travis/build/SciTools/iris/lib/iris/tests/system_test.py", line 72, in system_test_supported_filetypes
new_cube = iris.load_cube(saved_tmpfile)
File "/home/travis/build/SciTools/iris/lib/iris/__init__.py", line 225, in load_cube
cubes = _load_collection(uris, constraints, callback).merged().cubes()
File "/home/travis/build/SciTools/iris/lib/iris/__init__.py", line 165, in _load_collection
result = iris.cube._CubeFilterCollection.from_cubes(cubes, constraints)
File "/home/travis/build/SciTools/iris/lib/iris/cube.py", line 135, in from_cubes
for cube in cubes:
File "/home/travis/build/SciTools/iris/lib/iris/__init__.py", line 156, in _generate_cubes
for cube in iris.io.load_files(part_names, callback):
File "/home/travis/build/SciTools/iris/lib/iris/io/__init__.py", line 177, in load_files
for cube in handling_format_spec.handler(fnames, callback):
File "/home/travis/build/SciTools/iris/lib/iris/fileformats/netcdf.py", line 419, in load_cubes
engine = _pyke_kb_engine()
File "/home/travis/build/SciTools/iris/lib/iris/fileformats/netcdf.py", line 187, in _pyke_kb_engine
oldest_pyke_compile_file = min(tmpvar)
ValueError: min() arg is an empty sequence
```

This PR attempts to resolve this issue.