fix npe on AbstractKapuaConfigurableResourceLimitedService

[riccardomodanese]

solves #3020
Deep check for other possible npe must be done.

Signed-off-by: riccardomodanese riccardo.modanese@eurotech.com

Brief description of the PR.
conditional check before converting configuration value to int.

Related Issue
This PR fixes #3020

Description of the solution adopted
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Any side note on the changes made
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[codecov]

Codecov Report

Merging #3021 into develop will increase coverage by 0.04%.
The diff coverage is 50.00%.

@@              Coverage Diff              @@
##             develop    #3021      +/-   ##
=============================================
+ Coverage      58.06%   58.11%   +0.04%     
- Complexity      2595     2602       +7     
=============================================
  Files           1062     1062              
  Lines          23012    23012              
  Branches        2042     2043       +1     
=============================================
+ Hits           13362    13373      +11     
+ Misses          8699     8689      -10     
+ Partials         951      950       -1     

Impacted Files	Coverage Δ	Complexity Δ
...stractKapuaConfigurableResourceLimitedService.java	85.71% <50.00%> (-2.39%)	0.00 <0.00> (ø)
...ce/management/commons/call/DeviceCallExecutor.java	62.79% <0.00%> (-4.66%)	0.00% <0.00%> (ø%)
.../jbatch/persistence/JPAPersistenceManagerImpl.java	31.60% <0.00%> (+1.03%)	30.00% <0.00%> (ø%)
...apua/job/engine/jbatch/JobEngineServiceJbatch.java	51.85% <0.00%> (+1.85%)	8.00% <0.00%> (ø%)
...nnection/internal/DeviceConnectionServiceImpl.java	69.66% <0.00%> (+2.24%)	17.00% <0.00%> (+1.00%)
...atch/persistence/jpa/JpaExecutionInstanceData.java	90.24% <0.00%> (+2.43%)	23.00% <0.00%> (+1.00%)
.../DeviceManagementOperationRegistryServiceImpl.java	69.64% <0.00%> (+3.57%)	14.00% <0.00%> (+2.00%)
...rtz/persistence/KapuaQuartzConnectionProvider.java	100.00% <0.00%> (+12.50%)	5.00% <0.00%> (+1.00%)
...ob/engine/jbatch/exception/JobEngineException.java	66.66% <0.00%> (+33.33%)	2.00% <0.00%> (+1.00%)
.../engine/jbatch/exception/JobResumingException.java	50.00% <0.00%> (+50.00%)	1.00% <0.00%> (+1.00%)