fix setting null value of property in xml throws NullPointerException

[xuanuu]

解决如下问题：
当propertyString==null时，会抛空指针异常NullPointerException。例如：在xml的配置中，如果设置属性值为null， <property name="manager"></null></property>，即发生propertyString==null

[codecov-io]

Codecov Report

Merging #2646 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master   #2646      +/-   ##
===========================================
- Coverage     50.24%   50.2%   -0.04%     
+ Complexity     2061    2060       -1     
===========================================
  Files           413     413              
  Lines         12644   12644              
  Branches       1292    1292              
===========================================
- Hits           6353    6348       -5     
- Misses         5844    5846       +2     
- Partials        447     450       +3

Impacted Files	Coverage Δ	Complexity Δ
...work/apollo/spring/property/PlaceholderHelper.java	80% <0%> (-1.54%)	24 <0> (ø)
...mework/apollo/portal/component/PortalSettings.java	65.07% <0%> (-4.77%)	5% <0%> (ø)
.../apollo/internals/RemoteConfigLongPollService.java	79.62% <0%> (-0.62%)	27% <0%> (-1%)

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[coveralls]

Coverage decreased (-0.02%) to 53.765% when pulling 016db42 on xuanuu:master into 59d92f6 on ctripcorp:master.

[JaredTan95]

LGTM

[nobodyiam]

Looks good! It will be perfect if some unit tests are added to cover this scenario.