hotfix for significant-digits to not error if the input was 0 (#863)

In the last `pr` there was an error, that I just discovered and fixed.
Also I added test for this case in future, and some test for negative
numbers

```
0 | signfificant digits 2
 33 │
 34 │     let insignif_position = $n - 1 - ($num | math abs | math log 10 | math floor)
    ·                                       ──┬─              ────┬───
    ·                                         │                   ╰── 'math log' undefined for values outside the open interval (0, Inf).
    ·                                         ╰── value originates from here
 35 │
    ╰────
```

I'm sorry for hassle here 😞

stdlib-candidate/std-rfc/math/mod.nu

@@ -31,7 +31,16 @@ export def 'significant-digits' [
  _ => {$input}
  }
 
- let insignif_position = $n - 1 - ($num | math abs | math log 10 | math floor)
+ let insignif_position = $num
+ | if $in == 0 {
+ 0 # it's impoosbile to calculate `math log` from 0, thus 0 errors here
+ } else {
+ math abs
+ | math log 10
+ | math floor
+ | $n - 1 - $in
+ }
+
 
  # See the note below the code for an explanation of the construct used.
  let scaling_factor = 10 ** ($insignif_position | math abs)

stdlib-candidate/tests/math.nu

@@ -19,3 +19,11 @@ export def "test significant-digits-duration" [] {
 export def "test significant-digits-ints" [] {
  assert equal (123456 | math significant-digits 2) 120000
 }
+
+export def "test significant-digits-0" [] {
+ assert equal (0 | math significant-digits 2) 0
+}
+
+export def "test significant-digits-negative" [] {
+ assert equal (-1.23456789 | math significant-digits 5) (-1.2346)
+}