inconsistent behavior of `get()` methods of `PDFOutputDevice`s

[ashutoshvarma]

When calling get() with index out of page range RawImageOutput returns last page's image whereas TextOutput throws a IndexError.

Steps To Reproduce:-

d = x.Document("samples/simple1.pdf")
iout = x.RawImageOutput(d)
tout = x.TextOutput(d)

print(len(d))
print(iout.get(10))            # will return same as iout.get(0)
print(tout.get(10))            # will throw Index Error

Output:-

1
<PIL.Image.Image image mode=RGB size=1275x1651 at 0x7F179F573370>
Traceback (most recent call last):
  File "_test.py", line 15, in <module>
    tout.get(10)
  File "src/pyxpdf/textoutput.pxi", line 268, in pyxpdf.xpdf.TextOutput.get
    cpdef object get(self, int page_no):
  File "src/pyxpdf/textoutput.pxi", line 286, in pyxpdf.xpdf.TextOutput.get
    return self._get_bytes(page_no).decode('UTF-8', errors='ignore')
  File "src/pyxpdf/textoutput.pxi", line 209, in pyxpdf.xpdf.TextOutput._get_bytes
    if self._cache_texts[page_no] == None:
IndexError: list index out of range