PV3Reader: Corrected environment variable check

applications/utilities/postProcessing/graphics/PV3Readers/Allwmake

@@ -7,9 +7,9 @@ then
     case "$ParaView_VERSION" in
     3* | git)
 
-        if [ ! -d "${PV_PLUGIN_PATH}" ]
+        if [ ! -n "${PV_PLUGIN_PATH}" ]
         then
-            echo "$0 : PV_PLUGIN_PATH not a valid directory."
+            echo "$0 : PV_PLUGIN_PATH not a valid."
             exit 1
         fi