Use a filter to fail rendering undefined variables

Instead of globally using StrictUndefined, introduce a filter that
will trigger a render failure on undefined variables.

Use as `{{ value_json.someval | is_defined }}`.

homeassistant/helpers/template.py

@@ -387,6 +387,13 @@ def timestamp_utc(value):
         return value
 
 
+def fail_when_undefined(value):
+    """Filter to force a failure when the value is undefined."""
+    if isinstance(value, jinja2.Undefined):
+        value()
+    return value
+
+
 def forgiving_float(value):
     """Try to convert value to a float."""
     try:
@@ -402,12 +409,13 @@ class TemplateEnvironment(ImmutableSandboxedEnvironment):
         """Test if callback is safe."""
         return isinstance(obj, AllStates) or super().is_safe_callable(obj)
 
-ENV = TemplateEnvironment(undefined=jinja2.StrictUndefined)
+ENV = TemplateEnvironment()
 ENV.filters['round'] = forgiving_round
 ENV.filters['multiply'] = multiply
 ENV.filters['timestamp_custom'] = timestamp_custom
 ENV.filters['timestamp_local'] = timestamp_local
 ENV.filters['timestamp_utc'] = timestamp_utc
+ENV.filters['is_defined'] = fail_when_undefined
 ENV.globals['float'] = forgiving_float
 ENV.globals['now'] = dt_util.now
 ENV.globals['utcnow'] = dt_util.utcnow