Fixed an NPE when parsing malformed subjects

[bashir2]

Description of what I changed

See the added unit-test for an example malformed subject that triggers this.

E2E test

TESTED:

Added unit-test to reproduce the reported issue.

Checklist: I completed these to help reviewers :)

• I have read and will follow the review process.

• I am familiar with Google Style Guides for the language I have coded in.

  No? Please take some time and review Java and Python style guides.

• My IDE is configured to follow the Google code styles.

  No? Unsure? -> configure your IDE.

• I have added tests to cover my changes. (If you refactored existing code that was well tested you do not have to add tests)

• I ran mvn clean package right before creating this pull request and added all formatting changes to my commit.

• All new and existing tests passed.

• My pull request is based on the latest changes of the master branch.

  No? Unsure? -> execute command git pull --rebase upstream master

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 51.49%. Comparing base (3a6e9da) to head (70d193f).

Files with missing lines	Patch %	Lines
...java/com/google/fhir/analytics/FetchResources.java	71.42%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1291      +/-   ##
============================================
+ Coverage     51.47%   51.49%   +0.02%     
- Complexity      716      718       +2     
============================================
  Files            95       95              
  Lines          5916     5915       -1     
  Branches        790      791       +1     
============================================
+ Hits           3045     3046       +1     
+ Misses         2579     2578       -1     
+ Partials        292      291       -1     

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