added efficient assertions into the setIssuer

[mohammadJaliliTorkamani]

added new assertion:
assert issuer != null && !issuer.isEmpty() : "Issuer must not be null or empty.";

to the method "setIssuer" located at \src\main\java\com\google\firebase\auth\OidcProviderConfig.java

This assertion would prevent the application from running when the issuer variable is null or empty.

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[lahirumaramba]

Hi @mohammadJaliliTorkamani the existing code covers the same behavior. Did I miss anything here?