Explain why 58 can be rounded to the nearest hundred even though there is not a digit in the hundreds place?

For this case we have the following definition:

hundred place: number of three digits whose absolute value is greater than zero.

Then, we have the following rounding rule:

If the previous number is greater than or equal to five, the next number increases by one unit.

We then have the following number:

58

We observe that the number before a hundred place is equal to five.

Therefore, rounding off we have:

58 = 100

According to rounding rules we have 58 can be rounded to the nearest hundred:

100