Suggested LR not returned when min_grad_idx is 0 in plot() (#66)

`min_grad_idx` is a numerical value but it was not properly used to
determined whether suggested lr should be returned through the
following statement:
```python
if suggest_lr and min_grad_idx:
```

As @manuel-munoz-aguirre mentioned in issue #65, this IF-statement
will fail to return suggested lr when `min_grad_idx` is 0.

A proper solution for this issue should be explicitly checking
whether `min_grad_idx` is `None` or not (as it was initialized to
`None` in line 503).

tests/test_lr_finder.py

@@ -372,3 +372,21 @@ def test_suggest_lr():
     ax, lr = lr_finder.plot(skip_start=0, skip_end=0, suggest_lr=True, ax=ax)
 
     assert lr == 2
+
+    # Loss with minimal gradient is the first element in history
+    lr_finder.history["loss"] = [1, 0, 1, 2, 3, 4]
+    lr_finder.history["lr"] = range(len(lr_finder.history["loss"]))
+
+    fig, ax = plt.subplots()
+    ax, lr = lr_finder.plot(skip_start=0, skip_end=0, suggest_lr=True, ax=ax)
+
+    assert lr == 0
+
+    # Loss with minimal gradient is the last element in history
+    lr_finder.history["loss"] = [0, 1, 2, 3, 4, 3]
+    lr_finder.history["lr"] = range(len(lr_finder.history["loss"]))
+
+    fig, ax = plt.subplots()
+    ax, lr = lr_finder.plot(skip_start=0, skip_end=0, suggest_lr=True, ax=ax)
+
+    assert lr == len(lr_finder.history["loss"]) - 1

torch_lr_finder/lr_finder.py

@@ -532,7 +532,7 @@ def plot(
         if fig is not None:
             plt.show()
 
-        if suggest_lr and min_grad_idx:
+        if suggest_lr and min_grad_idx is not None:
             return ax, lrs[min_grad_idx]
         else:
             return ax