BUGFIX Call to a member function Categories() on null

[muskie9]

Resolves an issue where no blog selected causes a chained Categories() relation call to error when attempting to edit the block later in the cms.

I haven't been able to build a proper test case that demonstrates the fix, however in local testing the cms now loads if no Blog is selected on an existing block.

resolves #38

[codecov]

Codecov Report

Merging #39 (c1ad497) into 2.1 (2f3a0a5) will decrease coverage by 3.28%.
The diff coverage is 66.66%.

❗ Current head c1ad497 differs from pull request most recent head 29e80eb. Consider uploading reports for the commit 29e80eb to get more accurate results

@@             Coverage Diff              @@
##                2.1      #39      +/-   ##
============================================
- Coverage     85.10%   81.81%   -3.29%     
+ Complexity       14       13       -1     
============================================
  Files             1        1              
  Lines            47       44       -3     
============================================
- Hits             40       36       -4     
- Misses            7        8       +1     

Flag	Coverage Δ
php	?
unittests	81.81% <66.66%> (?)

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Impacted Files	Coverage Δ
src/Elements/ElementBlogPosts.php	81.81% <66.66%> (-3.29%)	⬇️

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