LogData.getTemplateContext() should be @NullableDecl

[bhamiltoncx]

LogData.getTemplateContext() is documented to return null for statements which do not require formatting:

  /**
   * Returns a template key for this log statement, or {@code null} if the statement does not
   * require formatting (in which case the message to be logged can be determined by calling
   * {@link #getLiteralArgument()}).
   */
  TemplateContext getTemplateContext();

However, this method is not marked @NullableDecl or @Nullable, so returning null triggers warnings from static analyzers.