Incorrect variable setting

**Describe the bug**
Hello,
I found an error with saving some variable values in Script Tasks. We use Groovy as the language for the Script Task. When setting the value of a variable through the execution.setVariable(), some values are replaced with null. These values are 0, " (empty string), false.

**Code**
execution.setVariable('test0', 0);
execution.setVariable('testFalse', false);
execution.setVariable('testEmptyString', '');

**Expected behavior**
test0 == 0
testFalse == false
testEmptyString == ''

**Real behavior**
test0 == null
testFalse == null
testEmptyString == null

**Additional context**
Flowable 6.7.2, PostgreSQL 13.0, Spring Boot.

Start by reproducing a Groovy Script Task using execution.setVariable() with 0, false, and an empty string on the reported Flowable 6.7.2 setup. Trace the execution.setVariable() entry point through variable persistence and add or run a regression test covering these values; done means each value is retrieved unchanged rather than as null.

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