slapos-sr-testing: Fix default parameters handling

If `only-sr` is not provided, all the SR tests should run.

Fixes merge request nexedi/slapos!1011

software/slapos-sr-testing/buildout.hash.cfg

@@ -15,4 +15,4 @@
 
 [template]
 filename = instance.cfg
-md5sum = 733643122fb75dfd7374b973a95fa2ea
+md5sum = 2c8b202b0c366e27dd6e6f456fb18256

software/slapos-sr-testing/instance.cfg

@@ -44,7 +44,7 @@ tests = {{ tests | indent(2) }}
 template =
   inline:{% raw %}
   {%- set only_sr = slapparameter_dict.get('only-sr') %}
-  {%- if not isinstance(only_sr, list) %}
+  {%- if only_sr and not isinstance(only_sr, list) %}
   {%-   set only_sr = [only_sr] %}
   {%- endif %}
   {%- set unittest_args = slapparameter_dict.get('unittest-args', ['discover', '-v']) %}