[processing] fix path handling (fix #14432)

(cherry picked from commit e873540)

Conflicts:
	python/plugins/processing/algs/otb/OTBUtils.py

python/plugins/processing/algs/grass/GrassUtils.py

@@ -86,7 +86,7 @@ def grassPath():
         if not isWindows() and not isMac():
             return ''
 
-        folder = ProcessingConfig.getSetting(GrassUtils.GRASS_FOLDER)
+        folder = ProcessingConfig.getSetting(GrassUtils.GRASS_FOLDER) or ''
         if not os.path.exists(folder):
             folder = None
         if folder is None:
@@ -107,7 +107,7 @@ def grassPath():
 
     @staticmethod
     def grassWinShell():
-        folder = ProcessingConfig.getSetting(GrassUtils.GRASS_WIN_SHELL)
+        folder = ProcessingConfig.getSetting(GrassUtils.GRASS_WIN_SHELL) or ''
         if not os.path.exists(folder):
             folder = None
         if folder is None:

python/plugins/processing/algs/grass7/Grass7Utils.py

@@ -83,7 +83,7 @@ def grassPath():
         if not isWindows() and not isMac():
             return ''
 
-        folder = ProcessingConfig.getSetting(Grass7Utils.GRASS_FOLDER)
+        folder = ProcessingConfig.getSetting(Grass7Utils.GRASS_FOLDER) or ''
         if not os.path.exists(folder):
             folder = None
         if folder is None:

python/plugins/processing/algs/otb/OTBUtils.py

@@ -101,24 +101,15 @@ def findOtbLibPath():
 
 
 def otbLibPath():
-    folder = findOtbLibPath()
-    if folder is None:
-        folder = ProcessingConfig.getSetting(OTB_LIB_FOLDER)
-    return folder
+    return ProcessingConfig.getSetting(OTB_LIB_FOLDER) or ''
 
 
 def otbSRTMPath():
-    folder = ProcessingConfig.getSetting(OTB_SRTM_FOLDER)
-    if folder is None:
-        folder = ""
-    return folder
+    return ProcessingConfig.getSetting(OTB_SRTM_FOLDER) or ''
 
 
 def otbGeoidPath():
-    filepath = ProcessingConfig.getSetting(OTB_GEOID_FILE)
-    if filepath is None:
-        filepath = ""
-    return filepath
+    return ProcessingConfig.getSetting(OTB_GEOID_FILE) or ''
 
 
 def otbDescriptionPath():