make it easier to disable default 'browser' settings

Previously it was necessary to set 'browser' to a non-empty, non-string
value to disable any default 'browser' value.
Now '-o browser=' or '-o browser=false' is enough.

gallery_dl/extractor/common.py

@@ -225,7 +225,9 @@ class Extractor():
         headers.clear()
         ssl_options = ssl_ciphers = 0
 
-        browser = self.config("browser") or self.browser
+        browser = self.config("browser")
+        if browser is None:
+            browser = self.browser
         if browser and isinstance(browser, str):
             browser, _, platform = browser.lower().partition(":")