Initialize a Diffs app state property `hasConflicts`

What does this MR do?

When we use properties of the state object, we should initialize them, too.

Uninitialized properties have strange behaviors - especially in unit tests - but may also not get reactive bindings in Vuex.

This MR initializes a property that we use, but have never initialized.

Screenshots (strongly suggested)

N/A, all backstage.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.