Description

Fix Type issue:
the options do not support null, and this PR fix it.

【problem description】
When new graph instance, there get type error:

You cannot assign type "HTMLElement | null" to type "HTMLElement | undefined". 
Type "null" cannot be assigned to type "HTMLElement | undefined". ts(2322) 
options.d.ts(15, 9): The required type is from the attribute "container", which is declared here on the "Partial<Manual>" type 
(property) container? : HTMLElement | undefined

Motivation and Context

1、Add type NullablePartial<T>, and use it in interface Manual which extended by Graph Option.

packages/x6/src/graph/options.ts

2、Unit test it in:

packages/x6/src/__tests__/graph/index.test.ts  // it('should return graph', () => {...})

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.