structures: don't leak invalid or empty strings when we warn
structures: don't leak invalid or empty strings when we warn

Fixes minor memory leak in unit tests caused by the recent changes.
Since we're expected to take ownership of the GValue in the structure
field struct here, we need to unset it if we don't use it.

ahmedammar/platform_external_gst_gstreamer,lovebug356/gstreamer,krieger-od/gstreamer,ahmedammar/platform_external_gst_gstreamer,mrchapp/gstreamer,drothlis/gstreamer,drothlis/gstreamer,cfoch/gstreamer,ensonic/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,drothlis/gstreamer,collects/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,krichter722/gstreamer,cablelabs/gstreamer,jpakkane/gstreamer,magcius/gstreamer,drothlis/gstreamer,magcius/gstreamer,shelsonjava/gstreamer,justinjoy/gstreamer,ylatuya/gstreamer,cablelabs/gstreamer,mrchapp/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,mparis/gstreamer,krieger-od/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,Distrotech/gstreamer,lubosz/gstreamer,ylatuya/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,jpakkane/gstreamer,justinjoy/gstreamer,centricular/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,lubosz/gstreamer,Distrotech/gstreamer,collects/gstreamer,centricular/gstreamer,ylatuya/gstreamer,ahmedammar/platform_external_gst_gstreamer,cablelabs/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,mparis/gstreamer,Distrotech/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,mrchapp/gstreamer,Distrotech/gstreamer,lubosz/gstreamer,Distrotech/gstreamer,collects/gstreamer,krieger-od/gstreamer,surround-io/gstreamer,cfoch/gstreamer,surround-io/gstreamer,centricular/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,justinjoy/gstreamer,ylatuya/gstreamer,cablelabs/gstreamer,justinjoy/gstreamer,ylatuya/gstreamer,collects/gstreamer,ensonic/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,magcius/gstreamer,jpakkane/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,centricular/gstreamer,cfoch/gstreamer,lubosz/gstreamer,krichter722/gstreamer,magcius/gstreamer,mparis/gstreamer,mrchapp/gstreamer,Lachann/gstreamer,ensonic/gstreamer,lovebug356/gstreamer,krieger-od/gstreamer,lovebug356/gstreamer,justinjoy/gstreamer,shelsonjava/gstreamer,magcius/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,krieger-od/gstreamer,mrchapp/gstreamer,surround-io/gstreamer,ahmedammar/platform_external_gst_gstreamer,StreamUtils/gstreamer,lubosz/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,jpakkane/gstreamer,drothlis/gstreamer,ahmedammar/platform_external_gst_gstreamer,collects/gstreamer,ensonic/gstreamer,mparis/gstreamer