Updating NumberToFloatingPointBitsSlow to handle the remaining parsed…

… fractional digits being zero. (dotnet/coreclr#27688)

Commit migrated from dotnet/coreclr@460b97a

src/libraries/System.Private.CoreLib/src/System/Number.NumberToFloatingPointBits.cs

@@ -484,7 +484,16 @@ private static ulong NumberToFloatingPointBitsSlow(ref NumberBuffer number, in F
             }
 
             AccumulateDecimalDigitsIntoBigInteger(ref number, fractionalFirstIndex, fractionalLastIndex, out BigInteger fractionalNumerator);
-            Debug.Assert(!fractionalNumerator.IsZero());
+
+            if (fractionalNumerator.IsZero())
+            {
+                return ConvertBigIntegerToFloatingPointBits(
+                    ref integerValue,
+                    in info,
+                    integerBitsOfPrecision,
+                    fractionalDigitsPresent != 0
+                );
+            }
 
             BigInteger.Pow10(fractionalDenominatorExponent, out BigInteger fractionalDenominator);