Handle non-existing variables and context entries gracefully

[saig0]

Describe the bug
If I access a non-existing variable then the expression fails. To avoid failure, I must do a null check first.

x
// failed to evaluate expression 'x': no variable found for name 'x'

The behavior is the same for path expressions. If I access a non-existing context entry, the expression fails.

{a: 1}.b
// failed to evaluate expression '{a: 1}.b': context contains no entry with key 'b'

Related to #540.
Related to #582.

To Reproduce
Steps to reproduce the behavior:

1. Evaluate the expression x
2. Verify that the expression fails to evaluate

Expected behavior
The expression should return null if a variable or a context entry doesn't exist.

x = null

{a: 1}.b = null

{a: 1}.b.c = null

null.b = null

If the path expression is applied to a filter and a context doesn't contain the entry, the resulting list should contain null for this item.

// non-member path filtering
[ { x: 1 }, { y: 2 } ].y = [ null, 2 ]

[ { x: 1 } ].y = [ null ]

The expected behavior is aligned with the DMN spec. It is verified by the TCK test cases:

• test(feel): verify non-existing member access dmn-tck/tck#541
• test(feel): verify non-member filter expression handling dmn-tck/tck#540

Environment

• FEEL engine version: 1.16.0
• Affects:
  • Camunda Automation Platform 7: [7.x]
  • Zeebe broker: [0.x]

[korthout]

ZPA triage:

• good issue for an upcoming mob programming session
• should be resolved before 8.3
• sizing as medium as it takes the team some time to resolve such issues