In histogram.py, only apply log scale if data has positive values (#8032)

Running pytest in Python 3.11 on a Debian Linux system produces these
warnings

```python
cirq-core/cirq/experiments/single_qubit_readout_calibration_test.py::test_estimate_parallel_readout_errors_no_noise
  /usr/local/google/home/mhucka/projects/github/cirq-pr-warnings/cirq-core/cirq/vis/histogram.py:123: UserWarning: Data has no positive values, and therefore cannot be log-scaled.
    set_semilog()

cirq-core/cirq/experiments/single_qubit_readout_calibration_test.py::test_estimate_parallel_readout_errors_no_noise
  /usr/local/google/home/mhucka/projects/github/cirq-pr-warnings/cirq-core/cirq/vis/histogram.py:124: UserWarning: Data has no positive values, and therefore cannot be log-scaled.
    set_lim(0, 1)

```

This PR modifies `integrated_histogram()` to only apply log scaling if
the data contains positive values, avoiding the warning from Matplotlib.

---------

Co-authored-by: Pavol Juhas <juhas@google.com>

Author: mhucka

cirq-core/cirq/experiments/single_qubit_readout_calibration_test.py

@@ -90,7 +90,7 @@ def test_estimate_single_qubit_readout_errors_with_noise() -> None:
 
 
 def test_estimate_parallel_readout_errors_no_noise() -> None:
-    qubits = [cirq.GridQubit(i, 0) for i in range(10)]
+    qubits = cirq.GridQubit.rect(10, 1)
     sampler = cirq.Simulator()
     repetitions = 1000
     result = cirq.estimate_parallel_single_qubit_readout_errors(
@@ -100,8 +100,6 @@ def test_estimate_parallel_readout_errors_no_noise() -> None:
     assert result.one_state_errors == dict.fromkeys(qubits, 0)
     assert result.repetitions == repetitions
     assert isinstance(result.timestamp, float)
-    _ = result.plot_integrated_histogram()
-    _, _ = result.plot_heatmap()
 
 
 def test_estimate_parallel_readout_errors_all_zeros() -> None:
@@ -176,7 +174,7 @@ def test_estimate_parallel_readout_errors_batching() -> None:
 
 
 def test_estimate_parallel_readout_errors_with_noise() -> None:
-    qubits = cirq.LineQubit.range(5)
+    qubits = cirq.GridQubit.rect(5, 1)
     sampler = NoisySingleQubitReadoutSampler(p0=0.1, p1=0.2, seed=1234)
     repetitions = 1000
     result = cirq.estimate_parallel_single_qubit_readout_errors(
@@ -188,6 +186,8 @@ def test_estimate_parallel_readout_errors_with_noise() -> None:
         assert 0.07 < error < 0.13
     assert result.repetitions == repetitions
     assert isinstance(result.timestamp, float)
+    result.plot_integrated_histogram()
+    result.plot_heatmap()
 
 
 def test_estimate_parallel_readout_errors_missing_qubits() -> None: