[text] slightly improve performance of 'extract' functions

by using 'None' instead of '0' as default 'pos' value
this only saves a few nanoseconds per call, but still

gallery_dl/text.py

@@ -100,7 +100,7 @@ def nameext_from_url(url, data=None):
     return data
 
 
-def extract(txt, begin, end, pos=0):
+def extract(txt, begin, end, pos=None):
     """Extract the text between 'begin' and 'end' from 'txt'
 
     Args:
@@ -125,7 +125,7 @@ def extract(txt, begin, end, pos=0):
         last = txt.index(end, first)
         return txt[first:last], last+len(end)
     except Exception:
-        return None, pos
+        return None, 0 if pos is None else pos
 
 
 def extr(txt, begin, end, default=""):
@@ -137,14 +137,14 @@ def extr(txt, begin, end, default=""):
         return default
 
 
-def rextract(txt, begin, end, pos=-1):
+def rextract(txt, begin, end, pos=None):
     try:
         lbeg = len(begin)
-        first = txt.rindex(begin, 0, pos)
+        first = txt.rindex(begin, None, pos)
         last = txt.index(end, first + lbeg)
         return txt[first + lbeg:last], first
     except Exception:
-        return None, pos
+        return None, -1 if pos is None else pos
 
 
 def rextr(txt, begin, end, pos=None, default=""):
@@ -156,7 +156,7 @@ def rextr(txt, begin, end, pos=None, default=""):
         return default
 
 
-def extract_all(txt, rules, pos=0, values=None):
+def extract_all(txt, rules, pos=None, values=None):
     """Calls extract for each rule and returns the result in a dict"""
     if values is None:
         values = {}
@@ -164,10 +164,10 @@ def extract_all(txt, rules, pos=0, values=None):
         result, pos = extract(txt, begin, end, pos)
         if key:
             values[key] = result
-    return values, pos
+    return values, 0 if pos is None else pos
 
 
-def extract_iter(txt, begin, end, pos=0):
+def extract_iter(txt, begin, end, pos=None):
     """Yield values that would be returned by repeated calls of extract()"""
     try:
         index = txt.index
@@ -182,7 +182,7 @@ def extract_iter(txt, begin, end, pos=0):
         return
 
 
-def extract_from(txt, pos=0, default=""):
+def extract_from(txt, pos=None, default=""):
     """Returns a function object that extracts from 'txt'"""
     def extr(begin, end, index=txt.index, txt=txt):
         nonlocal pos