Include placeholder tasks in get_running_builds

Pipeline jobs in in Jenkins 2.x appear as placeholder tasks for their
entire lifetime when queried via the API. The fix for #1659787
introduced completly ignoring placeholder tasks. Thus pipeline jobs are
never included in the list returned by get_running_builds.

This commit will change the behaviour of get_running_builds as follows:
  - Placeholder tasks will be included: So running pipeline jobs
    will be included.
  - Builds which do not have a build number yet will be ignored:
    This prevents the exception described in #1659787.

Change-Id: I30351ec78d6186b0674e3c08be00076b0ce2333e
Closes-Bug: #1750372
Related-Bug: #1659787

jenkins/__init__.py

@@ -1461,7 +1461,7 @@ class Jenkins(object):
                     raise
             for executor in info['executors']:
                 executable = executor['currentExecutable']
-                if executable and 'PlaceholderTask' not in executable.get('_class', ''):
+                if executable and 'number' in executable:
                     executor_number = executor['number']
                     build_number = executable['number']
                     url = executable['url']

tests/test_build.py

@@ -432,7 +432,7 @@ class JenkinsListRunningBuildsTest(JenkinsTestBase):
 
     @patch.object(jenkins.Jenkins, 'get_node_info')
     @patch.object(jenkins.Jenkins, 'get_nodes')
-    def test_placeholder_task_in_queue(self, nodes_mock, node_info_mock):
+    def test_unnumbered_task_in_queue(self, nodes_mock, node_info_mock):
         nodes_to_return = [{
             'name': "foo-slave", 'offline': False
         }]
@@ -448,13 +448,7 @@ class JenkinsListRunningBuildsTest(JenkinsTestBase):
                     "progress": -1
                 },
                 {
-                    'currentExecutable': {
-                        '_class': (
-                            'org.jenkinsci.plugins.workflow.support.steps.'
-                            'ExecutorStepExecution$PlaceholderTask$'
-                            'PlaceholderExecutable'
-                        )
-                    },
+                    'currentExecutable': {},
                     'currentWorkUnit': {},
                     'idle': False,
                     'likelyStuck': False,