get valid subscription handle even if cft failed

[iuhilnehc-ynos]

trafficstars

rcl need to continue to try rcl cft fallback if rmw implementation failed to call rmw cft.

[fujitatomoya]

@asorbini we would like to request the another review on this.

[iuhilnehc-ynos]

@asorbini

Thanks for your reply. No matter when you reply, it's never too late.

Is the ROS filtering syntax different than the standard DDS one?

No, I think it's the same for now, maybe RCL content filter fallback can support extra syntax in the future. The extra reason for patching this is the RCL content filter fallback could be used if rmw_connextdds is unsupported while using the rtime version.