Fix parsing of ISO8601 strings with empty period

Author: mgmarino

pandas/_libs/tslibs/timedeltas.pyx

@@ -620,10 +620,7 @@ cdef inline int64_t parse_iso_format_string(str ts) except? -1:
             if not len(unit):
                 number.append(c)
             else:
-                # if in days, pop trailing T
-                if unit[-1] == 'T':
-                    unit.pop()
-                elif 'H' in unit or 'M' in unit:
+                if 'H' in unit or 'M' in unit:
                     if len(number) > 2:
                         raise ValueError(err_msg)
                 r = timedelta_from_spec(number, '0', unit)
@@ -632,14 +629,14 @@ cdef inline int64_t parse_iso_format_string(str ts) except? -1:
                 neg = 0
                 unit, number = [], [c]
         else:
-            if c == 'P':
-                pass  # ignore leading character
+            if c == 'P' or c == 'T':
+                pass  # ignore marking characters P and T
             elif c == '-':
                 if neg or have_value:
                     raise ValueError(err_msg)
                 else:
                     neg = 1
-            elif c in ['D', 'T', 'H', 'M']:
+            elif c in ['D', 'H', 'M']:
                 unit.append(c)
             elif c == '.':
                 # append any seconds

pandas/tests/scalar/timedelta/test_constructors.py

@@ -228,6 +228,8 @@ def test_overflow_on_construction():
         ("P0DT0H0M0.001S", Timedelta(milliseconds=1)),
         ("P0DT0H1M0S", Timedelta(minutes=1)),
         ("P1DT25H61M61S", Timedelta(days=1, hours=25, minutes=61, seconds=61)),
+        ("PT1S", Timedelta(seconds=1)),
+        ("PT0S", Timedelta(seconds=0)),
     ],
 )
 def test_iso_constructor(fmt, exp):