Fix crash if LD input contact isn't connected

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/BeremizIDE.py", line 339, in OnMethod
obj.CTR.CallMethod('_'+meth)
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 1996, in CallMethod
getattr(self, method)()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 1134, in _Build
IECGenRes = self._Generate_SoftPLC()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 746, in _Generate_SoftPLC
if self._Generate_PLC_ST():
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 763, in _Generate_PLC_ST
self._getIECgeneratedcodepath())
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCControler.py", line 456, in GenerateProgram
self.ProgramChunks = GenerateCurrentProgram(self, self.Project, errors, warnings)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1762, in GenerateCurrentProgram
generator.GenerateProgram()
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 478, in GenerateProgram
self.GeneratePouProgram(pou_name)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 260, in GeneratePouProgram
program = pou_program.GenerateProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1718, in GenerateProgram
self.ComputeProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1028, in ComputeProgram
self.GenerateBlock(instance, block_infos, body, None)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1144, in GenerateBlock
expression = self.ComputeExpression(body, connections, executionOrderId > 0)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1373, in ComputeExpression
paths = self.GeneratePaths(connections, body, order, to_inout)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1344, in GeneratePaths
elif isinstance(result[0], list):
IndexError: list index out of range


Close #70

How Do I Submit A Good Pull Request?
----------------------------------

It's highly recommended to write nice and clean python code. Beremiz
project tries to follows most of PEP-8 recommendations. They are
automatically checked on every push and merge by Bitbucket pipelines.

To avoid pushing "unclean" code, i's recommended to add one of the following
commands to pre commit Mercurial hook into .hg/hgrc configuration file.

Unfortunately script can't distinguish between real commit and shelve. If you
are using shelve (or maybe some other affected commands), it's recommended to
use pre-<command> and post-<command> hooks to create flags to skip checks on
some operations.


```
[hooks]
pre-shelve.linter = touch .hg/skiphook
post-shelve.linter = rm .hg/skiphook
pretxncommit.linter = ./tests/tools/check_source.sh --only-changes
```
or the same done using Docker container, so result will be the same as
on Bitbucket pipeline.

```
[hooks]
pre-shelve.linter = touch .hg/skiphook
post-shelve.linter = rm .hg/skiphook
pretxncommit.linter = hg status -m -n -a -n -I '**.py' --change $HG_NODE > files.lst && docker run --volume=$PWD:/beremiz --workdir="/beremiz" --volume=$PWD/../CanFestival-3:/CanFestival-3 --memory=1g --entrypoint=/beremiz/tests/tools/check_source.sh skvorl/beremiz-requirements --files-to-check files.lst
```