fix(flex-offset): correct return type when using with 'strictNullChecks'

[Teamop]

Currently, when using @angular/flex-layout@latest(7.0.0-beta.21) with "strictNullChecks": true, got error:

because the default type here is:

export declare class FlexOffsetStyleBuilder extends StyleBuilder {
    buildStyles(offset: string, parent: FlexOffsetParent): {
        [x: string]: string;
        'margin-top'?: undefined;    // error here
    } | {
        'margin-top': string;
    };
}

If with "strictNullChecks": false, got no error, so here just correct the return type of buildStyles.

Fix #928

@CaerusKaru @ThomasBurleson

[Teamop]

Don't know why the checks don't run...

[CaerusKaru]

@Teamop try it now. Also please enable strictNullChecks in the demo-app so we can test for this moving forward.

[Teamop]

@CaerusKaru done

[CaerusKaru]

LGTM

[CaerusKaru]

@Teamop thank you for patching this!

[iNViTiON]

Thank you. Fast fix.

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