Merging pillars when one is empty gives empty result

Description of Issue/Question

I'm trying to merge pillar data, as suggested in https://github.com/saltstack/salt/issues/28394#issuecomment-158659657 . However, if the data I try to merge in happens to be empty (as a result of template rendering), the end result is completely empty, ignoring any previous data.

Setup

pillar/top.sls:

base:
  '*':
    - common
    - modules

pillar/common.sls:

program:
  modules:
    00-definitions.conf: ~
    01-cleanup.conf: ~

pillar/modules.sls:

program:
  modules:
    {% if 'test' in grains['roles'] %}
    10-dostuff.conf: ~
    {% endif %}

From this, I expect a minion with the grain roles: [test] to get the pillar {'00-definitions.conf': None, '01-cleanup.conf': None, '10-dostuff.conf': None}, while other minions should get {'00-definitions.conf': None, '01-cleanup.conf': None}.

However, what actually happens is that minions without the role test get nothing at all:

# salt --no-colour 'salt*' grains.get roles   
saltminion:
    - test
saltmaster:
    - saltmaster

# salt --no-colour 'salt*' pillar.get program
saltminion:
    ----------
    modules:
        ----------
        00-definitions.conf:
            None
        01-cleanup.conf:
            None
        10-dostuff.conf:
            None
saltmaster:
    ----------
    modules:
        None

Changing pillar_source_merging_strategy from smart to recurse gives the same result.

Versions Report

Salt Version:
           Salt: 2015.8.8

Dependency Versions:
         Jinja2: 2.8
       M2Crypto: Not Installed
           Mako: Not Installed
         PyYAML: 3.11
          PyZMQ: 15.2.0
         Python: 2.7.11 (default, May  7 2016, 22:38:59)
           RAET: Not Installed
        Tornado: 4.3
            ZMQ: 4.1.4
           cffi: 1.6.0
       cherrypy: Not Installed
       dateutil: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
        libgit2: Not Installed
        libnacl: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.4.7
   mysql-python: Not Installed
      pycparser: 2.14
       pycrypto: 2.6.1
         pygit2: Not Installed
   python-gnupg: Not Installed
          smmap: Not Installed
        timelib: Not Installed

System Versions:
           dist: gentoo 2.2 
        machine: x86_64
        release: 4.5.5-hardened-r1
         system: Gentoo Base System 2.2 

Reproduce the issue with the three pillar files and the two grain configurations shown in the report, then trace the pillar source-merging path for an empty rendered mapping. Done means the common pillar data remains present when the conditional pillar renders empty, for both the smart and recurse strategies.