FIX: Handling of PDF empty string properties

[carlwilson]

• removed null check/fix in Property class which might have unintended side effects for other modules (it may be the right thing to do, but more thought and testing are required);
• removed the succession of null pointer catches in PDFModule;
• added specific handling of null String values in PDFModule.addStringProperty as this seems to apply the fix in the correct place with fewer potential side effects;
• added PDF regression test file for empty string properties; and
• copied affected integration test results files from 1.26.

This is a reworking of the contribution from @samalloing in PR #782 Fixes #809

[codecov[bot]]

Codecov Report

Base: 46.33% // Head: 46.22% // Decreases project coverage by -0.10% :warning:

Coverage data is based on head (c1719c7) compared to base (e3e4e35). Patch coverage: 57.72% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## integration #820 +/- ## ================================================= - Coverage 46.33% 46.22% -0.11% - Complexity 1054 1056 +2 ================================================= Files 57 57 Lines 9079 9057 -22 Branches 1612 1607 -5 ================================================= - Hits 4207 4187 -20 + Misses 4333 4330 -3 - Partials 539 540 +1 ``` | [Impacted Files](https://codecov.io/gh/openpreserve/jhove/pull/820?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openpreserve) | Coverage Δ | | |---|---|---| | [.../main/java/edu/harvard/hul/ois/jhove/Property.java](https://codecov.io/gh/openpreserve/jhove/pull/820?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openpreserve#diff-amhvdmUtY29yZS9zcmMvbWFpbi9qYXZhL2VkdS9oYXJ2YXJkL2h1bC9vaXMvamhvdmUvUHJvcGVydHkuamF2YQ==) | `45.76% <23.07%> (+3.33%)` | :arrow_up: | | [...src/main/java/edu/harvard/hul/ois/jhove/Utils.java](https://codecov.io/gh/openpreserve/jhove/pull/820?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openpreserve#diff-amhvdmUtY29yZS9zcmMvbWFpbi9qYXZhL2VkdS9oYXJ2YXJkL2h1bC9vaXMvamhvdmUvVXRpbHMuamF2YQ==) | `62.16% <61.81%> (-7.68%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openpreserve). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openpreserve)

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