fix(strace): fix the test for an associative array

The original test seems to try to check whether the array `syscalls`
is non-empty, but this does not work for associative arrays.  The
expansion ${array_name} references ${array_name[0]}, but
${array_name[0]} does not necessarily exist even when the array is
non-empty when the array is an associative array or a sparse array.

We can explicitly check the number of elements.

completions/strace

@@ -42,7 +42,7 @@ _strace()
                                     $syscall =~ ^__NR_(.+) ]] &&
                                     syscalls[${BASH_REMATCH[1]}]=1
                             done 2>/dev/null </usr/include/asm/unistd.h
-                            if [[ ! $syscalls ]]; then
+                            if ((${#syscalls[@]} == 0)); then
                                 local unistd arch=$(command uname -m)
                                 if [[ $arch == *86 ]]; then
                                     unistd=/usr/include/asm/unistd_32.h