[SPARK-29888][SQL] new interval string parser shall handle numeric with only fractional part

### What changes were proposed in this pull request?

Current string to interval cast logic does not support i.e. cast('.111 second' as interval) which will fail in SIGN state and return null, actually, it is 00:00:00.111.
```scala
-- !query 63
select interval '.111 seconds'
-- !query 63 schema
struct<0.111 seconds:interval>
-- !query 63 output
0.111 seconds

-- !query 64
select cast('.111 seconds' as interval)
-- !query 64 schema
struct<CAST(.111 seconds AS INTERVAL):interval>
-- !query 64 output
NULL
````
### Why are the changes needed?

bug fix.
### Does this PR introduce any user-facing change?

no
### How was this patch tested?

add ut

Closes apache#26514 from yaooqinn/SPARK-29888.

Authored-by: Kent Yao <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: yaooqinn

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/IntervalUtils.scala

@@ -496,6 +496,17 @@ object IntervalUtils {
           state = TRIM_BEFORE_SIGN
         case TRIM_BEFORE_SIGN => trimToNextState(b, SIGN)
         case SIGN =>
+          currentValue = 0
+          fraction = 0
+          // We preset next state from SIGN to TRIM_BEFORE_VALUE. If we meet '.' in the SIGN state,
+          // it means that the interval value we deal with here is a numeric with only fractional
+          // part, such as '.11 second', which can be parsed to 0.11 seconds. In this case, we need
+          // to reset next state to `VALUE_FRACTIONAL_PART` to go parse the fraction part of the
+          // interval value.
+          state = TRIM_BEFORE_VALUE
+          // We preset the scale to an invalid value to track fraction presence in the UNIT_BEGIN
+          // state. If we meet '.', the scale become valid for the VALUE_FRACTIONAL_PART state.
+          fractionScale = -1
           b match {
             case '-' =>
               isNegative = true
@@ -505,14 +516,13 @@ object IntervalUtils {
               i += 1
             case _ if '0' <= b && b <= '9' =>
               isNegative = false
+            case '.' =>
+              isNegative = false
+              fractionScale = (NANOS_PER_SECOND / 10).toInt
+              i += 1
+              state = VALUE_FRACTIONAL_PART
             case _ => return null
           }
-          currentValue = 0
-          fraction = 0
-          // Sets the scale to an invalid value to track fraction presence
-          // in the BEGIN_UNIT_NAME state
-          fractionScale = -1
-          state = TRIM_BEFORE_VALUE
         case TRIM_BEFORE_VALUE => trimToNextState(b, VALUE)
         case VALUE =>
           b match {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/IntervalUtilsSuite.scala

@@ -106,6 +106,7 @@ class IntervalUtilsSuite extends SparkFunSuite {
     checkFromString("-1.5 seconds", new CalendarInterval(0, 0, -1500000))
     // truncate nanoseconds to microseconds
     checkFromString("0.999999999 seconds", new CalendarInterval(0, 0, 999999))
+    checkFromString(".999999999 seconds", new CalendarInterval(0, 0, 999999))
     checkFromInvalidString("0.123456789123 seconds", "Error parsing interval string")
   }