Remove leading empty lines

When running a cell, the leading indentation on the first line is
removed from all of the cell's lines. This causes indentation errors
on subsequent lines in case the first line consists entirely of
whitespace matching a valid level of indentation that is greater than
the least indented line.

This change avoids the issue by first removing any leading,
whitespace-only lines.

Closes  #11594

IPython/core/inputtransformer2.py

@@ -18,6 +18,19 b' import warnings'
 
 _indent_re = re.compile(r'^[ \t]+')
 
+def leading_empty_lines(lines):
+    """Remove leading empty lines
+
+    If the leading lines are empty or contain only whitespace, they will be
+    removed.
+    """
+    if not lines:
+        return lines
+    for i, line in enumerate(lines):
+        if line and not line.isspace():
+            return lines[i:]
+    return lines
+
 def leading_indent(lines):
     """Remove leading indentation.
 
@@ -510,6 +523,7 b' class TransformerManager:'
     """
     def __init__(self):
         self.cleanup_transforms = [
+            leading_empty_lines,
             leading_indent,
             classic_prompt,
             ipython_prompt,

IPython/core/tests/test_inputtransformer2_line.py

@@ -86,3 +86,31 b' def test_leading_indent():'
     for sample, expected in [INDENT_SPACES, INDENT_TABS]:
         nt.assert_equal(ipt2.leading_indent(sample.splitlines(keepends=True)),
                         expected.splitlines(keepends=True))
+
+LEADING_EMPTY_LINES = ("""\
+    \t
+
+if True:
+    a = 3
+
+b = 4
+""", """\
+if True:
+    a = 3
+
+b = 4
+""")
+
+ONLY_EMPTY_LINES = ("""\
+    \t
+
+""", """\
+    \t
+
+""")
+
+def leading_empty_lines():
+    for sample, expected in [LEADING_EMPTY_LINES, ONLY_EMPTY_LINES]:
+        nt.assert_equal(
+                ipt2.leading_empty_lines(sample.splitlines(keepends=True)),
+                expected.splitlines(keepends=True))