res_geolocation: Fix segfault when there's an empty element

Fixed a segfault caused by var_list_from_loc_info() encountering
an empty location info element.

Fixed an issue in ast_strsep() where a value with only whitespace
wasn't being preserved.

Fixed an issue in ast_variable_list_from_quoted_string() where
an empty value was considered a failure.

ASTERISK-30215
Reported by: Dan Cropp

Change-Id: Ieca64e061a6d9298f0196c694b60d986ef82613a

include/asterisk/strings.h

@@ -265,6 +265,7 @@ enum ast_strsep_flags {
   \param sep A single character delimiter.
   \param flags Controls post-processing of the result.
   AST_STRSEP_TRIM trims all leading and trailing whitespace from the result.
+  If the result containes only whitespace, it'll be passed through unchanged.
   AST_STRSEP_STRIP does a trim then strips the outermost quotes.  You may want
   to trim again after the strip.  Just OR both the TRIM and STRIP flags.
   AST_STRSEP_UNESCAPE unescapes '\' sequences.