Example:
ProcessInstance pi = runtimeService.startProcessInstanceByKey("testProcess",
Collections.<String, Object>singletonMap("var", Variables.longValue(null)));

assertNotNull(runtimeService.createProcessInstanceQuery().variableValueNotEquals("var", (long) 123).singleResult());

The assertion fails although the process instance should match, as it contains a variable called "var".

In addition, test and specify the following cases:

• variable is untyped null => query for untyped null (not equals and equals)
• variable is typed null => query for untyped null (not equals and equals)
• variable is untyped null => query for typed null (not equals and equals)
• variable is typed null => query for typed null (not equals and equals)
• variable is typed null => query for typed value (not equals and equals)
• variable is untyped null => query for typed value (not equals and equals)
• variable is typed value => query for untyped null (not equals and equals)
• variable is typed value => query for typed null (not equals and equals)

Roman and I agreed on the following scheme:

• with untyped null, i.e. variableValueEquals("var", null), both typed and untyped null are matched
• with typed null, i.e. variableValueEquals("var", Variables.longValue(null)), only typed null of type long is matched
• with typed value != null, i.e. i.e. variableValueEquals("var", 123L), only typed values with that value are matched
• for all these queries holds that variableValueNotEquals returns the complementary set