[SalesForce] LWC: how to handle null situation before the data is loaded from controller

e.force:navigateToComponent is deprecated. I wont recommend using it.

Instead use lightning:navigation , you generate state(aka URL params) by setting page references.

<lightning:navigation aura:id="navService"/>

JS:

 var navService = cmp.find("navService");
    // Sets the route to /lightning/o/Account/home
    var pageReference = {
        type: 'standard__component',
        attributes: {
            componentName: 'c:testContainer'
        },
        state : {
            c__recordId : cmp.get('v.recordId');
        }
    };

    // Set the URL on the link or use the default if there's an error
    var defaultUrl = "#";
    navService.generateUrl(pageReference).then($A.getCallback(function(url) {
        window.open(url,'_blank'); // This will open in new tab with pageReference with state/URL params you sent while creating instance of pageReference
    }

In your client-side aura controller, set the pageReference attribute for the Component . Set the URL on your link using the generateUrl() method, which is useful for opening links in a new tab

Now you have to make your LWC component listen to pageReference and get state(the recordId that we sent)

you can do that in your lwc by declaing @wire(CurrentPageReference)

import { CurrentPageReference } from 'lightning/navigation';

export default class TestLWC {


     // Injects the page reference that describes the current page
    @wire(CurrentPageReference)
    currentPageReference; 

    get recordIdFromState(){
        return this.currentPageReference &&
            this.currentPageReference.state.c__recordId; // Once you have recordId you can call apex
    }
}

Per the documentation here you want to mark your @wire methods differently.

Also from the doc:

In the wire adapter’s configuration object, prefix a value with $ to reference a property of the component instance. The $ prefix tells the wire service to treat it as a property of the class and evaluate it as this.propertyName. The property is reactive. If the property’s value changes, new data is provisioned and the component rerenders.

The right syntax is:

 @wire(getDE, { a: '$a', b: '$b', c: '$c' })
    deWired;

You also shouldn't need to call it in your connectedCallback. You can also edit your deWrired syntax to perform callback actions

@wire(getDE, { a: '$a', b: '$b', c: '$c' })
    deWired({ error, data }) {
        if (data) {
            this.d = data.d;
            this.e = data.e;
        } else if (error) {
            this.error = error;
        }
    }