Allow #if $datasets #end if patern for file lists

Otherwise the following exception occurs:
```
Traceback (most recent call last):
  File "/home/wolffj/src/galaxy/lib/galaxy/jobs/runners/__init__.py", line 191, in prepare_job
    job_wrapper.prepare()
  File "/home/wolffj/src/galaxy/lib/galaxy/jobs/__init__.py", line 858, in prepare
    self.command_line, self.extra_filenames, self.environment_variables = tool_evaluator.build()
  File "/home/wolffj/src/galaxy/lib/galaxy/tools/evaluation.py", line 435, in build
    raise e
AttributeError: 'DatasetListWrapper' object has no attribute 'value'
```

This is because DatasetListWrapper inherits `__bool__` from ToolParameterValueWrapper
which defines bool as
```
    def __bool__(self):
        return bool(self.value)
    __nonzero__ = __bool__
```
while DatasetListWrapper has no `value` attribute.

Fixes #6314 reported by
@joachimwolff (many thanks for the detailed report!).

lib/galaxy/tools/wrappers.py

@@ -352,6 +352,11 @@ def to_dataset_instances(dataset_instance_sources):
     def __str__(self):
         return ','.join(map(str, self))
 
+    def __bool__(self):
+        # Fail `#if $param` checks in cheetah if optional input is not provided
+        return len(self) > 0
+    __nonzero__ = __bool__
+
 
 class DatasetCollectionWrapper(ToolParameterValueWrapper, HasDatasets):