subpicture: don't call text renderer with no text

The text renderers and text to speech module don't produce anything if
there's no text. No need for defensive programming in each module.

modules/text_renderer/freetype/freetype.c

@@ -971,9 +971,6 @@ static subpicture_region_t *Render( filter_t *p_filter,
                          const subpicture_region_t *p_region_in,
                          const vlc_fourcc_t *p_chroma_list )
 {
-    if( !p_region_in->p_text )
-        return NULL;
-
     filter_sys_t *p_sys = p_filter->p_sys;
     subpicture_region_t *region = NULL;
     bool b_grid = (p_region_in->text_flags & VLC_SUBPIC_TEXT_FLAG_GRID_MODE) != 0;

modules/text_renderer/nsspeechsynthesizer.m

@@ -104,9 +104,6 @@ static subpicture_region_t *RenderText(filter_t *p_filter,
         filter_sys_t *p_sys = p_filter->p_sys;
         const text_segment_t *p_segment = p_region_in->p_text;
 
-        if (!p_segment)
-            return NULL;
-
         for ( const text_segment_t *s = p_segment; s != NULL; s = s->p_next ) {
             if ( !s->psz_text )
                 continue;

modules/text_renderer/sapi.cpp

@@ -129,9 +129,6 @@ static int RenderTextMTA(filter_t *p_filter,
     struct filter_sapi *p_sys = static_cast<struct filter_sapi *>( p_filter->p_sys );
     const text_segment_t *p_segment = p_region_in->p_text;
 
-    if (!p_segment)
-        return VLC_EGENERIC;
-
     for (const text_segment_t *s = p_segment; s != NULL; s = s->p_next ) {
         if (!s->psz_text)
             continue;

modules/text_renderer/svg.c

@@ -332,10 +332,6 @@ static subpicture_region_t *RenderText( filter_t *p_filter,
                        const subpicture_region_t *p_region_in,
                        const vlc_fourcc_t *p_chroma_list )
 {
-    /* Sanity check */
-    if( !p_region_in->p_text )
-        return NULL;
-
     for( size_t i=0; p_chroma_list[i]; i++ )
     {
         if( p_chroma_list[i] == VLC_CODEC_BGRA )

src/video_output/vout_subpictures.c

@@ -321,6 +321,8 @@ static subpicture_region_t *SpuRenderText(spu_t *spu,
 {
     spu_private_t *sys = spu->p;
     assert(subpicture_region_IsText( region ));
+    if ( region->p_text == NULL )
+        return NULL;
 
     vlc_mutex_lock(&sys->textlock);
     filter_t *text = sys->text;