Add null check for signature inputs outputs

[ahmedsabie]

Fixes #6945.
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This change is 

[pyu10055]

thank you for fixing this, the style seems to be bit weird, can you take a look, otherwise LGTM.

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @ahmedsabie and @mattsoulanille)

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tfjs-converter/src/executor/graph_executor.ts line 648 at r1 (raw file):

    const result: NamedTensorMap = {};
    for (const inputName in inputs) {
      const tensor = this._signature ?.inputs ?.[inputName];

there should be no spaces before the ?

[mattsoulanille]

LGTM with the same nit as Ping had about not having spaces in optional chaining.

[kashyap-das]

When is this going to be released? I am getting this error in production env. Need quick fix here please. Thanks in advance