core-util: Improve pa_replace() behaviour

- Assert that the search string isn't empty.
 - Add test.
 - Improve documentation.

src/pulsecore/core-util.c

@@ -3194,6 +3194,7 @@ char *pa_replace(const char*s, const char*a, const char *b) {
 
     pa_assert(s);
     pa_assert(a);
+    pa_assert(*a);
     pa_assert(b);
 
     an = strlen(a);