Fix max_attempts=''

That's what studio defaults to, and recent changes made it break.
Added a few tests to make sure it doesn't happen again.

common/lib/capa/capa/tests/test_html_render.py

@@ -11,6 +11,20 @@ from . import test_system
 
 class CapaHtmlRenderTest(unittest.TestCase):
 
+    def test_blank_problem(self):
+        """
+        It's important that blank problems don't break, since that's
+        what you start with in studio.
+        """
+        xml_str = "<problem> </problem>"
+
+        # Create the problem
+        problem = LoncapaProblem(xml_str, '1', system=test_system)
+
+        # Render the HTML
+        rendered_html = etree.XML(problem.get_html())
+        # expect that we made it here without blowing up
+
     def test_include_html(self):
         # Create a test file to include
         self._create_test_file('test_include.xml',

common/lib/xmodule/xmodule/capa_module.py

@@ -135,8 +135,8 @@ class CapaModule(XModule):
             self.grace_period = None
             self.close_date = self.display_due_date
 
-        max_attempts = self.metadata.get('attempts', None)
-        if max_attempts is not None:
+        max_attempts = self.metadata.get('attempts')
+        if max_attempts is not None and max_attempts != '':
             self.max_attempts = int(max_attempts)
         else:
             self.max_attempts = None

common/lib/xmodule/xmodule/tests/test_capa_module.py

@@ -144,6 +144,8 @@ class CapaModuleTest(unittest.TestCase):
                             "Factory should be creating unique names for each problem")
 
 
+
+
     def test_correct(self):
         """
         Check that the factory creates correct and incorrect problems properly.
@@ -784,6 +786,12 @@ class CapaModuleTest(unittest.TestCase):
         module.lcp.done = True
         self.assertTrue(module.should_show_save_button())
 
+    def test_no_max_attempts(self):
+        module = CapaFactory.create(max_attempts='')
+        html = module.get_problem_html()
+        # assert that we got here without exploding
+
+
     def test_get_problem_html(self):
         module = CapaFactory.create()