taglists: make _get_{string|pointer} return FALSE for NULL values
taglists: make _get_{string|pointer} return FALSE for NULL values

Make gst_tag_list_get_string() return FALSE for NULL strings and
empty strings, and gst_tag_list_get_pointer() return FALSE for
NULL pointers, like we do with dates and buffers.

Fixes #560345.

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