quote: handle numm and empty strings in sq_quote_buf_pretty

The sq_quote_buf_pretty() function does not emit anything
when the incoming string is empty, but the function is to
accumulate command line arguments, properly quoted as
necessary, and the right way to add an argument that is an
empty string is to show it quoted, i.e. ''. We warn the caller
with the BUG macro is they pass in a NULL.

Reported by: Junio Hamano <[email protected]>
Signed-off-by: Garima Singh <[email protected]>

Author: garimasi514

quote.c

@@ -48,6 +48,16 @@ void sq_quote_buf_pretty(struct strbuf *dst, const char *src)
 	static const char ok_punct[] = "+,-./:=@_^";
 	const char *p;
 
+	/* In case of null tokens, warn the user of the BUG in their call. */
+	if (!src) 
+		BUG("Cannot append a NULL token to the buffer");
+	
+	/* Avoid dropping a zero-length token by adding '' */
+	if (!*src) {
+		strbuf_addstr(dst, "''");
+		return;
+	}
+
 	for (p = src; *p; p++) {
 		if (!isalpha(*p) && !isdigit(*p) && !strchr(ok_punct, *p)) {
 			sq_quote_buf(dst, src);

t/t0014-alias.sh

@@ -37,4 +37,11 @@ test_expect_success 'looping aliases - internal execution' '
 #	test_i18ngrep "^fatal: alias loop detected: expansion of" output
 #'
 
+test_expect_success 'run-command parses empty args properly, using sq_quote_buf_pretty' '
+    GIT_TRACE=1 git frotz a "" b " " c 2>&1 |
+    sed -ne "/run_command:/s/.*trace: run_command: //p" >actual &&
+    echo "git-frotz a '\'''\'' b '\'' '\'' c" >expect &&
+    test_cmp expect actual
+'
+
 test_done